Slow acceleration at times from stop starts and

After changing the Alternator Belt on Oct.10,2009, my car 2001 Nissan Sentra GXE frequently stopped at redlight signals but I was able to restart immediately.My mechanic raised the idle running rpm but after running for about 40 miles the problem persisted.Last week he changed the sensor but that hasn't helped.He says that the computer does not indicate the problem and is not able to pinpiont where the actual defect lies.Could you please help in solving this as I am moving out of the country and trying to sell the car.Thanks.
K.V.

Have you done a tune up? Tune up may be the only problem next would be fuel filter then fuel pump. With all the new additives in gas fuel filters don't last as long as they used to.

Hello. Thanks for choosing Fixya! Begin by changing the fuel filter and the air filter. If that doesn't solve your problem take your Nissan to any autoparts store or shop that can put it on a scanner to see what other issues the car has. THanks for choosing fixya! Regards, Joe