Stalls in idle, 1982 VW Rabbit

For about a month, my 1982 Rabbit (fuel injected) has been stalling almost every time I take my foot off the gas. The only time it doesn't is for the first 5 minutes or so in the morning. Can anyone tell me some things that A) I can check/repair myself, or B) A mechanic could diagnose for free?

I don't know if this makes any difference, but the car has never had a gas cap in the 4 months that I have owned it. Could someone have put something in my gas tank that would cause this maybe. Or, could the missing cap be a problem in and of itself, even though everything was fine for 3 months?

My main question is this: How likely is it that the repairs it needs are going to be expensive?
We only meant for this car to get us through the summer anyway, so am not interested in doing any major repairs. If it seems likely that the repairs are going to be expensive, I don't want to pay a mechanic just to tell me that either :-)

One other thing if anyone knows. Where can I buy a brake fluid reservoir cap for this car? I've searched and searched with no luck!

Thanks so much for any advice!

(I don't know the mileage as the odometer was broken when we bought it, stuck at 119,000 miles. As for the engine size, I think it is a 1.7. It's just the basic model Rabbit, not convertible or anything)