Mountain Biking - Help! My chain pops off

Hey all, I have an '04 Jamis exile with stock components. Whenever I'm peddling with a lot of torch in low gear (up hills), my chain pops off. Most of the time it comes off the front derailer completely. Other times it skips to a higher gear in the rear cassette. My bike's been tuned up, no one can find anything wrong with it. What's wrong? Thanks.

...torch in low gear...
I assume you mean 'torque'. Thats happend to my friends before while riding up a 6km hill, I got told why it happend, but i've totally forgotten now. It also happens when you try and change the front gears, mainly because the front chainrings are taking all the pressure or something, and the chain can slip? Not too sure....

I meant "torque". Sorry.

That is the exact same problem I have having with my marin bobcat trail. If anyone has the answer please let me know....would be much appreicated

-Check your front derailleur limit adjustments.
-Check your rear derailleur cable tension (you need a bit more probably).
-Your chainwheels (front gears) are made of Jello.
My guess is the first two.

You didn't mention if you were shifting at the time, but try not shifting under heavy load (torque) either.

Your front derailleur inside limit screw isn't properly adjusted. Shift into the big cog in back and the granny gear. Set the inside limit to where it starts to scrape the chain then back it off a 1/4 turn.

As far as your chain climbing gears in the back. You need to adjust the cable tension. If that doesn't work, you may be experiencing problems from a slightly bent derailleur hanger.

If you adjust your cable tension and it works in lets say the bottom half of the gears, but hangs up in the top half, it's usually a bent hanger.

Your shop should have a derailleur alignment tool. Have them check it for you. If it's not too bent, they can usually bend it back. However, realize that a hanger is made of aluminum and doesn't hold up too well to repeated bending, rebending.

What about a chain tensioner?

Thanks for all your help, I'll look into this.