Sorry, should have mentioned earlier - to determine for sure, just put on ANY other freewheel. If the cogs mesh OK then the 15th cog must be the problem. No, it's not frame flex or a tight link, otherwise it would not be limited to only one cog.
Originally Posted by
cminter
You need to read closer "the drive train only will run in the 15t cog with out skipping", now since the rest of us clearly know nothing any better ideas?
Stated wrong but still the correct troubleshooting step. Should have read "If the cogs mesh on a different freewheel then the freewheel is the problem." I never said others know nothing but I have not seen a logical explanation of how the other supposed causes could cause the specific symptoms of skipping under pressure in all cogs except one. The OP needs to switch out the freewheel, otherwise everyone keeps on guessing.