2001 V70 A/C shuts off when stopped in traffic

[rxfrchng]

I have a very odd problem with my 2001 V70. The A/C works fine when I'm on the highway, but when I'm either in stop and go traffic or driving at slower speeds (40 mph or less), the a/c compressor seems to shut off (no cold air). When this happens, the interior fan still blows, but there is no cold air. If I turn off the fan or the a/c for a few minutes, the a/c will blow cold air again, but only if I'm back up at a higher rate of speed (+65 mph). Any help would be greatly appreciated. Thanks.

[Jot]

If you can borrow a refill guage for the system you can check the pressure of the system.