I am new to this forum and hope I can get some help. I just finished building myself a K2 Animal (98 frame) with mostly new components but I re-used my old shimano 7sp drivetrain (ran out of cash). The drivetrain has 4 seasons on it. I did replace the chain with a new shimano IG chain. The original chain was an IG also. On the stand, everything runs perfectly smooth, as well as on the street, as long as there is moderate to no load. As soon as I apply some heavy torque, the chain skips at the back end. The chain doesn't appear to be coming off the cog at either side. It almost looks as though it is literally skipping a tooth. I can get this to happen on the 11, 13, 15, & 18t cogs. I didn' t try on the bigger ones, I would need to be on the trail (or at least a hill) to maintain any torque. I would greatly appreciate any help.

I have somewhat the same problem, but I can see it is the cassette, did you put the old cassette on there?

You've probably worn the cogs on the old cassette, most likely from using your chains too long before replacment The bushings in the chain will wear after 500 to 2,000 miles changing the distance from link to link, (stretching). When this happens the links put more pressure on fewer and fewer links because the contact points are no longer fully engaged, wearing the cogs. Your favorite gears are always the ones to go first because you use them more. The wear can be noted by either a slightly widened area were the chain contacts the cog under pressure and by the cog taking a "saw tooth" or "sharkfin" shape. Park tools makes some gauges to measure your cogs and chains for wear so you can replace them before they're worn enough to cause damage, you can measure the chain with a ruler , but it is a pain in the a** to do. If you stay on top of the chain measurement/wear the cogs will last for a very long time. If you go for the gauges get the chain gauge first, for most people it's the most useful.

You chain-rings can/will suffer the same damage as the cogs but luckily not as often as there are more of them to spread the load

I agree with Pat you need to replace your cassette. Anytime you replace your chain say after a year or two plan on doing the cassette as well.

agree it's probably the cassette. I replace my chain about 3 times a year. It greatly extends the life of the Chainrings and cassette which are much more expensive. Depending on your amount of riding you may have to do it less frequently.