Classic problem -inconsistent/erratic idle, occasional stalling from idle especially when coming to a stop. Most likely causes are a dirty/sticky/worn idle air control (IAC) valve or a dirty throttle body followed by a misadjusted throttle plate stop screw, misadjusted throttle position sensor (TPS) or misadjusted throttle body linkage -in that order of likelihood. Clean the flame trap and all associated vacuum hoses/ports at the same time. The base idle on your car is not adjustable as in older models. There are other possibilities that can contribute to a poor idle, but many are associated with generally poor running. Check the 700/900 FAQ feature section here (upper right pull down menu) for more information.

Mechanic said nothing is wrong? Sorry, those are common problems with Volvo engines (LH-Jetronic in particular) otherwise they wouldn't be in the FAQ. Now you can at least point him in the right direction, but it's not hard to check all this and correct it yourself using information right here on the Brickboard.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day