Have the same problem on the 106 XSI i have at the mo, was told it could be the plugs or leads causing a slight misfire, or some damp has got into a plug. Try squirting some water displacer like WD40 on the plugs and leads or changing them altogether. I tried all this though and still does it, even hooked it up to a diagnostic machine at main dealer with no joy!!!!! Still runs fine when warmed up so not that bothered. Very frustrating though!!! ???