your torque converter is going out. very common issue with these cars. im on my third tranny. try and find a used one with low miles and swap it in.

if you can work on the car yourself you'll save ALOT of cash. if not you'll probably spend more on maintenance than what you did on the car itself.

your gauge being all red and unreadable is also a very common issue.

DIY:
Official DIY 2000 Audi S4 LCD Instrument Cluster Pixel Repair "No Hot Air Rework Station Needed" - AudiWorld Forums

there is also a guy on audizine.com, named M3POWER, that does the repairs as well. PM him on there.