So I bought a 1991 lugged Trek 750 black/white/yellow. After cleaning it up and setting the derailleurs, I had this problem in the small cog where it kept skipping. The bike had replacement Shimano brake shifters model ST-EF65-7R/L with Suntour XCE rear and Suntour front derailleur. I used the Park youtube video to refresh my memory, but in the end I still had the problem. I brought it to my local mechanic and he said the Shimano and Suntour indexing were not compatible. He made some adjustments and it seemed to work reasonable. Today I did the complete wax job in preparation of putting the bike on Craigslist (I have 2 Trek 750s, I'm keeping the 1996 Ice Blue one), then I decided to make sure the recent tune was good. Unfortunately it's exactly like when I brought it to the mechanic. Under load it skips in the small cog. Since my mechanic is off on Monday, I went ahead and took the Shimano STX front and rear derailleur off from my other Trek 750 with idea of using them to make an all Shimano compatible system. I found out the tubing on the 1996 & 1991 Trek 750s are not the same size, so the front clamp for the Shimano STX from the 1996 Trek 750 was too big. So I put the Suntour front derailleur from the 1991 back on. After several hours, I could not get it to work correctly and was wondering if the Shimano STX rear was the problem? Does anyone have some insight about what to do next? Thanks, Nelson