trek hardtail 3900 shifts under load uphill, got this in nov 09 new, rode it a couple of times, no problems,the first day above 40 I had it out and rode it long and hard. Going uphill with weight on pedals only, it shifted and I almost had my first crash of the year. Its happened quite a few times since then, any ideas? Thnx.
First of all check out the info here, especially Alex's video. Than print the instructions and go through the tune up process. Park Tool site has a good process for setting up the dérailleur. Basically start by setting the hi/low positions so the rear dérailleur is straight down in both top and bottom position, the front so it just clears the inside and outside than start tune up on the center front sprocket using the cable barrels a 1/2 turn at a time.
Not enough info given is it the front or rear??
If you do not have a bike stand use rope to hang your bike from rafters, bike hooks or whatever you can so you can pedal by hand and see what the problem is.
Never Give Up!!!