I had the same problem on my car. The temporary "solution" was to disconnect the full-throttle switch. I think I have basically decided that the warmup regulator gives too much fuel with the full throttle switch, and I may end up going back to full throttle switch only (no boost sensitive warmup regulator).
This may not be a well known fact, but it's true...the CIS system in the Volvo, though it did have a boost sensitive warmup regulator, it did NOT use this feature during "normal running" -- only when the car was cold. A vacuum valve blocked the boost signal to the WUR once engine temp got a little bit warm...
I have found that full throttle switch + 240-T fuel distributor gives good results.