Belinda,
That's a petty common occurrence with Volvos (and many other cars). All sorts of causes and cures. Most of them involve fiddling with the timing (advance or retard a little) and/or the carbs (a little richer or a little leaner). Sometimes readjusting the valves makes a difference, sometimes cleaning the carbon from the pistons (manually, or by feeding some water into the intake manifold). Some cars, for example some VWs, came equipped with automatic fuel cut offs. If nothing else cures it, rigging something like that would work, but one would have to be pretty desperate. Punch "dieseling" into the SEARCH box for more reading on the subject. I usually just stomp on the gas pedal, and that seems to work for me, usually, but may not for you.

Bob S.
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"62 PV544 (B20, M41), "71 142E, "93 240 Classic Wagon.