Hi John,

I had a similar problem about 18months ago on my '96 850.  It turned out to
be a "temperature sensitive" pressostat switch.  That's the pressure switch
on the firewall that controls the compressor on/off cycling.

When it got hot it would fail to turn the compressor on.  Generally, if the
a/c was turned on while the car was cold, say first thing in the morning, it
would work ok even in 40deg C outside temp.  The cold return gas from the
evaporator kept it cool.  But if stuck in traffic or the engine turned off
for even a few minutes, the exhaust manifold would heat it to where it would
fail to operate.

You can check for this by pulling off the pressostat plug and putting a
short circuit across the two pins in the plug (with the motor running),
while your a/c is not working properly.  The compressor should run and the
a/c work.

The pressostat can be replaced without de-gassing the a/c.  It has a
"schrader" type valve under it.  Mine was really tight though.  It also cost
about AUD120 (from Volvo).

My high temp switch also failed a couple of weeks later, so I bypassed it
and things are still ok after 18 months.

regards
barry