Hey i'm runnin the 420A motor, and everyonce in a while my tranny will pop out of third when i touch it. almost as if it is barely in third and when i go to shift into 4th, it pops right out; this obviously causes it to be in neutral and the engine revs high... could this be a bad synchros in the tranny? has anyone ever had this problem?

 

sounds like a syncro to me...