911 short run engine stalls

I just bought my first 911 (1990 C4 conv.), after I sold my 968. The engine was rebuild at Motor Meister in California with the bigger bore package to 3.9 liter engine. The engine now has 10.000 mls. after rebuild. The engine stalls sometimes after a short run, when I come to a stop sign. It starts right up with no problems. I have no problems on longer trips, but in the city, stop and go. I have good gas mileage and the car does not smoke, and idles good. I do all the maintenance myself, and all the components were replaced, after rebuilding the engine. Fuel pump is ok, and fuel filter was replaced. I tried to get the problem solved with a Porsche dealer, but no luck. Any suggestions. Thanks.