Sputtering Turbo
 

I recently bought a 1998 eclipse GS-T. It has many mods ranging from evo 3 GT turbo, big front mount intercooler, electronic boost controller, turbo timer, 550 injectors, and SAFC II to list a few. One issue that seems to pop up though is when I reach 4500 RPM the turbo starts sputtering and the engine loses power. It tries to fight through but the turbo isn't consistent after the 4500 mark. Im not sure if the SAFC is just out of tune or what the problem is. When its running 15 lbs of boost the sputtering begins earlier in the RPM range (3500ish). Any help would be awesome, Ill be tossing the datalogger on it this week to check the AFR's and see if that leads me to any answers.

Try a boost leak test.

check your plugs/wires, then like he said boost leak....and yes you really need to watch the AFR's