Volvo Repair: Volvo 740 stalls at 4000 rpm, volvo 740, knock sensor

Question
I have a 1989 740 with a wierd problem.  After warming up ( this does not happen until it has been operating for a while ) when accellerating through 4000 rpm the engine stalls and hangs at 300-500 rpm.  The only remedy is to shut the engine down ( with the ingition ) and restart. It always restarts, no problem.  Otherwise the car runs just fine.  Any suggestions on where to look?

Answer
after this happens without shutting it off I would check the ignition timing with a timing light.there could be a problem with the knock sensor which is retarding the ignition timing.if this is ok then the best way to check it would be to be able to see each sensor of the fuel system with a scan tool and see what is happening when it does this.