I had a similar problem in the Iowa summers only, especially on hot days. After first trying my backup air mass meter, I tried disconnecting the electrical connection to the Cold Start Injector (plug is under intake manifold - two-prong electrical connector), suspecting that it was running the the engine too rich upon warm start for some unknown reason. Deactivating the cold start injector (by unplugging the electrical connector only) immediately solved the problem.

So, a person on Brickboard explained that the computer was probably getting inaccurate information from the coolant temperature sensor, and injecting extra fuel unnecessarily at warm start. I then put in a new coolant temperature sensor, plugged back in the cold start injector electrical connector, and everything is now fine. The computer is now getting accurate information about the coolant temperature.

For my 1991 240 the coolant sensor is:
Volvo Temperature Sensor - 1346030
It has a two-prong electrical connection.

Both the cold start injector and coolant sensor are somewhat hard to access underneath the intake manifold. Unplugging the cold start injector electrical connector isn't too difficult, but, my car has EGR (California version 240), so I had to remove the EGR to gain access to the coolant sensor with a deep socket. There are some good videos online regarding the process of changing it out. It's a bit tough but it can be done with the intake manifold in place.

Your problem might be caused by some other things, but the steps above solved my seemingly similar engine problem.

Joe