Check engine light

I have a 1999 Audi A4, 1.8T. Today I drove to the store and everything was fine. When I started my car to come home, it immediately was vibrating and the Engine light was flashing. It seemed to lack power - I had the distinct feeling that it wasn't getting much power and if I pushed the accelerator it wouldn't go. However, it did run, but felt really 'wimpy'. The vibrating continued, it was like the car was ready to just die out. I got onto the highway where the vibrating continued. I switched to Tiptronic (Is that the right word? When you shift yourself?) and there was no difference, but when the car kicked into 5th gear the vibrating immediately stopped and the light stopped flashing, but it did remain on solid. When I got home I turned the car off and turned it back on - they symptons weren't there but the check engine light was still on. I did 'check the engine' (visually)- there wasn't anything obvious going on. Thanks for any help!