98 GTI 2.0l Power Loss
 

In November I purchased a 98 GTI 2.0l and it performed great, smooth and quick acceleration. I made the unfortunate mistake of taking it to a dealer when my brake lights had a short. Come to find out the short was in the rp light housing, I had them repair it and also had them replace my air filter and fp fog light. After I received the car it performed terribly, and had minor body damage due to a lift jack. After bringing the car back to have the body repaired, I had them them look into the performance issue to no avail (no reports faults logged). The car occasionaly studders starting from a complete stop after the car is hot, I have also noticed that my tailpipe has a small black deposit around the entire pipe (running rich perhaps). I also noticed I have a full return of power after I unhook the battery and reconnect it (to reset sensors), but only for 5 mins or so then back to being weak. The car struggles a little after 2200 rpms, and fuel econ remains at about 26.8mpg around town. The only thing that comes to mind is the maf sensor, but I am wondering if anyone has any other thoughts before I go and replace it. Thanks in advance.

Sounds like it's as simple as needing a new battery.