This entire issue has me absolutely flustered. Hopefully someone can give me some guidance.

1998 Mitsubishi 3000GT, SL. Manual. 150k mi.

First a bit of history:
I bought this car a few months ago. Got my first oil change and things were good for a few weeks. Then the check engine light came on, but when I went back to the original owner, it had gone back off. The light would come on, stay on for a few driving sessions, go back off, and repeat. We couldn't find the problem.

Then it won't start for me one morning. When looking under the hood, I noticed that my battery connectors looked awful like someone had repeatedly struck the terminals to get the connectors on. Wires are exposed and the connectors could be easily slid off the terminals. The battery looked pretty clean and new though. I assumed that this was probably what was causing my check engine light problems, so I promptly got the car jumped and took it to Pepboys so they could replace the connectors for me. I also paid for a general system check (battery, alternator, starter) and they told me that my battery needed replacing. When I asked if they did the test before or after the connector replacement, they were very vague, so I decided not to replace my battery at the time. Additionally, they tell me that my battery doesn't have a tie-down. They can't fix it for me, so nothing happens regarding this.

When the car was done, I picked it up in the parking lot. Seconds after I pull out of my spot, the car dies. I restart it, cruise through the parking lot, and it dies again. Repeat. I notice that the idle is weird, but after some time it stabilizes and I drive back home.

The car is fine again for a few days and then one morning, it won't start on me again. It kind of wants to turn over... and then starts clicking rapidly. Frustrated, I start pumping the gas petal while cranking, and surprisingly, it starts up. I find that it is generally after sitting overnight, the way to get it started is to pump the gas petal. During lunch breaks, after each morning's ordeal, the car starts regularly, without extra "assistance."

And now...
Yesterday morning, it won't start again. It is just making a loud buzzing sound when I try to start it. Remembering that Pepboys suggested replacing my battery, I pick one up from Walmart. I throw it in, try starting it, and things are good. I didn't take it out yesterday, assuming things are now finally ok.
This morning, car starts up fine as well. On the route to work, while sitting at a stop light, the car suddenly dies. I restart it and keep going. While decelerating to a stop, the car dies again. I restart it. Die. Accelerating. Die. It is reminiscent of that ordeal in the Pepboys parking lot, but it is much more terrifying when this is happening on the highway. I have made it into work, but I'm dreading 5:00 when I need to drive back home.

I'm really sorry for being so lengthy, but I don't want to leave out any details in case they are pertinent. I'm at wit's end about this considering driving this car off a bridge. Please help?

 

Sounds like worn out brushes in the alternator, or defective alternator itself. It could also be the worst - ECU issues due to jump-starting and loose battery terminals. This kind of thing happens when battery terminals are loose and/or disconnectedwhile the engine is running -stress to electrical system of entire car. I'm not much familiar with your car. You need to get a good picture by obtaining wiring diagrams and shop manual to analyze the condition.

 

Crap. Well, that's scary...
The Pepboys diagnostic specifically said that my alternator and starter are fine, so this worst-case-scenario seems like quite a possibility. I've never dealt wth ECU issues before... Does this mean that my car is on its way to becoming a 2 ton paperweight?

I'm not sure as to what to do now. Should I take it somewhere again and completely rule out that its an alternator issue? Are their any other areas/components that I should look into? Should I just stick to plan A and drive it off a bridge?

Wiring diagrams and shop manuals are probably waaay over my head [:'(]

 

You said you had a check engine light, what were the codes and also you said there was a loud buzzing noise, were you able to pinpoint where the noise was coming from?

 

I never successfully got a code because all three times I went to get the code pulled, the light had turned off that same or previous day. It wouldn't consistently stay on. However, the check engine light ceased after I got the battery connectors replaced. I haven't seen it come on again after that.

The loud buzzing sound occurred when I turned the key to start the car yesterday morning. I wasn't thinking clearly, as I was worried about how I would get to work, but I did notice that it was from the passenger side of the engine bay (also where my battery is located). At the same time, I don't know how reliable or helpful this information is because I didn't even pop the hood or poke around. Sorry, I wish I had a better answer

 

If the check engine light comes on again get it scanned asap, it should give you a code even if it goes off on the way to get it scanned. It will be stored in memory.

Do you have a dvom so you can verifiy if the car is charging b/c I dont trust any kids who work at pep boys. Sounds like it could be several things you just have to eliminate different systems so you can find the problem.