Sounds like a typical case of worn wiper arm tracks in the air flow meter. Over the years, your car has spent alot more time at part throttle compared to 3/4 to full throttle. Thus that portion of the conductive material has worn away and is providing poor signals to the DME.
Everything you need to fix this issue is here....
Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com
You can also check you tube for instructional videos on the repair.