Does this happen when the fuel tank is full? Perhaps fuel is moving away from the pickup.

Another possibility is a poor electrical connection somewhere. I have an '88 740 that has a bad pin in its air mass meter plug. I discovered the problem when I found that the car idled just fine, just barely moved if you tried to accelerate in Drive, but went like hell in Reverse!

The weight of the wire was just enough to break the connection when the car tried to accelerate forward. Interestingly enough, my service advisor was not too surprised to hear of the bad pin, and he told me how to pull the pin out and bend the lock tang a bit to effect a repair.

When I suffered a bad connection in our '90 Turbo, I was advised to clean all connections and put then together with dielectric grease in them. No troubles since.

BTW - Even though you do not have a 740, you might want to read the FAQs, as some of the troubles that have been documented there will happen to 850s too.

Have you tried a search?
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3 8s & 2 7s 725,000 miles total