Hello!
I have very strange problem with my volvo 850. it is 10v and 1996. It runs perfectly and very smooth when it is cold. But when it gets warm it starts to get rough and doesn't response to throttle. and eventually dies.
I have changed like everything. ecu, throttle position sensor, coil, distributor, plug wires, fuel pump, fuel pump relay, camshaft and crankshaft sensors, relay and the other thing above radiator. and lots of other stuff.

I believe the timing goes wrong or injectors don't get command. Because fuel pressure stays on and it gives spark. Today i rev it lot and exhaust and catalysator turned red and it threw pieces of catalysator out. And it sounds like timing is wrong. But what goes wrong when it heats up.

Volvo 850 10v -96

 

Sounds like your temperature sensor. The one on the thermostat housing.

 

I don't know. But, I really doubt that timing would do that....what I'd expect is the fuel mixture changing and the temp-sensor would be on the top of my suspects list.