Hi all,

I recently pulled an older mtb out of retirement and upgraded parts of the drivetrain, and am now unable to get it shifting smoothly no matter what I try. It is a 1996 Trek 6500 with Shimano LX crankset and LX front/rear deraillers (these were installed back in 2000, but we're not used much prior to retiring the bike shortly thereafter). Also installed Sachs grip shift at that time. Triple front, 8-speed rear.
The recent drivetrain upgrades include a SRAM 870 chain and SRAM 8-speed rear cassette. It will not shift properly (either double shift when downshifting, or skips gears, depending on how I adjust the barrel screw). Since it has been unused for the last decade, I am not sure if the problem relates to that, or due to incompatibility between drivetrain components. I have fiddled with it dozens of times over seversl rides with no luck...Now I am not sure what to try/replace next!
Note: I also replaced the cable and housing hoping that was the culprit, but it did not help the situation at all.

Any thoughts appreciated!
Shawn