I wouldn't call this a phenomena.
You must observe exactly what is happening, you may have to systematically go through many tests, fuel pressure, check for vacuum leaks, check plugs, wires, cap, rotor, coil, etc.... ECU.
If it seem quite intermittent, it may be difficult to diagnose, but if it starts getting worse, it may get to be quite obvious what the problem is. Some problems like this could be easily diagnosed on a chassis dyno if it happens quite often, you need a load, and a dyno is the only way to get one.
I'd try another qualified Volvo mechanic to give you a suggestion.
BTW: You shouldn't post to newsgroups in HTML, many news readers won't pick this up, and people may not see your post. Use text only.
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Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19
