gear slipping
 

i have a 19 year old bike which i have been riding for a few months and when i apply a lot of torque ( power up hills) in a low gear it slips out and moves to a higher gear. Any ideas of what it could be? thanks

check your rear dérailleur adjustment . sound like the dérailleur is tired and need replacing .

Could also be the cable tension is too low.
Check Sheldon Brown's site about ghost shifting. As I recall, he covers the phenomenon pretty well.

If the shifters are indexed try adjusting the shift cable tension.
If the shifters are friction then try adding friction at the shifter.

Al

Also check for stiff links in the chain