My '92 940 had this problem about six months ago. The RPM sensor was changed and all has been well until this past week.
I pulled the OBD codes last night but the fault conditions described by the codes do not match what is happening with the car.
In Box A, on socket 2, I got a 3-1-1 code. That shows to be a "signal missing from speedometer" fault condition. The symptoms it should have according to that code are idling problems. Note, the car cranks but will not start.
Again in Box A, this time on socket 6, I got a 1-4-3 code. The fault condition is knock sensor signal absent or faulty. The symptoms of that are poor acceleration, low power, high fuel consumption.
Unfortunately, my Volvo mechanic cannot get the car in before September 20. I would really like to ge the car running before then.
Any ideas on where else I need to look?

Thanks.