Volvo Repair: 1990 Volvo 240DL SW keeps turning off!, volvo 240dl, volvo dealer

Question
Hello,
I have a 1990 Volvo 240 station wagon, manual shift.  The number of miles is hard to determine, because the odometer broke at about 50k; I would estimate it has about 150k miles.
Starting in 9/05, it started producing an unusual behavior.  While I was driving, the car would turn off.  The power steering would disappear, I would be unable to accelerate (or even rev the engine), and the Check Engine light would usually turn on.
Between 9/05 and 4/06, I dealt with the problem by coasting with the clutch in, turning the key fully off, turning the car back on while still coasting, and shifting back into gear.  I noticed the behavior about maybe once every 40 miles or so.  It didn't seem that the behavior occurred at a certain speed, or after a certain amount of warming up, or after bumps in the road.
Within the last month, the problem has gotten worse.  The behavior occurs maybe every 10 miles, and I can't turn the car back on for at least a minute after it goes off.  So when it happens, I must pull over and stay on the side of the ride for a while---it's become a fairly serious safety issue and I don't take the car on highways.  Sometimes, the behavior occurs after the car comes to a complete stop; this didn't often happen before
I took it to a Volvo dealer.  The car's computer reported a problem with the RPM meter, which was replaced.  This didn't stop the behavior.  After a further three days of testing, the dealership reported it couldn't figure out what was wrong with the car.  I'm sorry, I don't know precisely which tests they performed.
What do you think could be the problem with the car?
Thanks so much,
Will

Answer
Will, this can be anything from the engine speed sensor to the fuel relay.  Many different parts control different things.  I would have the dealer deal with it since they didnt fix the problem.  This is one of those things that I would need to do tests to diagnose the car properly.  Roger