I have a 1989 740 SW non-turbo. Periodically, about once or twice it week, it will suddenly begin to run terribly. There will be no power, idle will fluctuate between 500 & 1200, barely creeps along with the pedal to the floor. After 15-20 seconds of this there will be a couple of pops or backfires and then it takes off like a bat out of hell. It usually happens when I start it after sitting for awhile after being fully warmed up but it has happened while at crusing speed on the freeway. I've changed plugs, put in a new fuel relay, tested the fuel pressure, tested for vacuum leaks, put in a new O2 sensor and put on a new catalytic converter. The check engine light never comes on during any of this foolishness and yes I've checked to make sure it is working. Any thoughts out there. Thanks in advance for any ideas you may have.