This is sort of off the wall, but I got it from an independent Volvo guy with years of experience here and in Norway. He says this symptom (which usually blooms in the Fall each year, and is mainly an LH 2.2 thing) is due to accumulated deposits on the intake valves and in the general valve area.

The theory is that these deposits soak up a portion of what the ECU intended as extra fuel for cold-running. After a few minutes, as the deposits become saturated and the engine warms a bit, the mixture then becomes adequate for the conditions.

The only cure seems to be a good fuel system cleaner, such as BG-44K or the more readily available Techron (on Volvo's 30K maintenance schedule at one time).

It would seem that an undetected air leak could also cause the problem, maybe when "helped" by a sluggish O2 sensor? How old is yours?
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.