I had the same symptoms you described with a 87 260E. I did all the different brands of octane booster, had my timing checked. I hope you don't have the same problem I did which is worn cam lobes. The last two lobes for cylinder six were badly worn.
The car made loud knocking noises only on like you said moderate to hard acceleration. When I would baby it, no knocking at all or it was really soft I couldn't hear it over the radio. It also felt like I didn't quite have full power and the engine was lagging just a little. After changing the cam, everything went back to normal and I haven't had a problem since.
I hope this helps.
1987 260E Charcoal Grey 157,000 Miles
1987 260E Forest Green 120,000 Miles