While a friend was on vacation I worked on his Trek 7200, with Alivio Derailleur and Shimano Twist Shifters. I replaced the chain and cassette, lubed the freehub, regreased the jockey rollers, cleaned, lubed and adjusted the rear der. The cable moved the der freely and easily on the stand. So, when riding in 4th or 5th gear, it shifts to smaller cogs by itself. Intentional shifting is crisp and precise.

On the stand, the shifting was quick, smooth and beautiful. No hints of problems at all. My guess (not necessarily an educated one) is that the new chain is stiff and this will work itself out after a few weeks. I installed the chain with the factory lube which is pretty thick. Any ideas? bk

I've had this happen to me before. I had adjusted the cable, and i wrapped the cable around the fixing bolt the opposite way of what it was supposed to be. Try attaching the cable on the other side of the fixing bolt.

Another possibility is that you need to add tension to the rear derailleur cable. Try 1/2 turn or so counter-clockwise on the barrel adjuster. Or it might be a slightly bent derailleur hanger.

Al