power goes out downshifting

My boyfriend and I just bought an 89 Cabrio. We love it....but it has a problem. We drove to Frankfurt ( about an hour away,) and the car ran just fine, When we tried to leave, the car died whenever I took the foot off the accelerator. We had it towed to a garage, and they fixed it temperarily. Unfortunantly, the mechanic wasn't familiar with American spec cars. Now, it has a VERY low idle (about 500) and it loses power briefly whenever I turn and downshift. If I give it gas, it gains power again. Also, it stalls whenever I reverse. The mechanics are not very helpful, but if I knew what the problem is, I could fix it myself using my handy vw book. Or, I could translate the parts into German and explain it to them. I just have no clue... Is the idle set too low. Or maybe it's a fuel injector problem....Any help would be much appreciated!! Thank you.

Have you checked/cleaned the auxillary air regulator? Also, I can't remember if CIS-L uses a throttle position switch for closed throttle.