I had the same problem with a 1.6 mk3 I used to have, when you approach a junction or just put the clutch down the revs would drop and usually catch themselves again (if engine was warm), however when cold it would stall.
I spent loads trying to get it sorted, had a new map sensor, new ECU, but still didn't solve the problem.
Once I bought my 8v GTI, after about a month the same thing started happening, not as bad, my mate (an ex VW technician) cleaned the inside of the throttle body - Never happened since!