I’ll try to make this quick. My 90 Volvo GLE Wagon with the B234 will not start at times. When it is cold it starts up every time. If drive it, shut it down an immediately try it, it starts. If I shut down and wait 5, 10, 15 minutes or the time it takes to get gas, it may not start. The starter cranks but the engine does not start, and sometimes when I turn the key over I get nothing, not even a click. The best thing to do is wait awhile where it starts just fine. When the engine is running, it runs fine and never quits. I looked on Swedishbricks.com and got info on the same issues and changed what it said to change; the starter was tested and was fine, I replaced the RPM sensor and the ignition switch.
So I got frustrated and took it into Volvo. Where they are having a hard time reproducing the failure. There’s got to be a bad sensor somewhere that is sending out a bad signal when the engine is hot. Maybe the Ignition control module or ignition control unit?
Just don’t know. This is the first big issue I have ever had with this car.

Given that you replaced the switch and the rpm sensor, the two other likely failure modes are the fuel injection relay and the radio suppression relay, both of which can fail and cause no-warm-start conditions. See the 700/900FAQ under "features" above for more details. You can resolder both relays or buy new ones.

Maybe it is related to this

http://www.brickboard.com/FAQ/700-900/EngineFuelinjection.htm#Radio_Suppression_Relay

If the relay is getting hot and shorting internally that meybe the cause.