Originally Posted by powers2b
I guarantee the issue is not rider weight related.
It is possible that the crank is bottoming out on the spindle taper.
This is usually brought about by a ham fisted mechanic overtightening the crank bolt.
If this is indeed the case you have a couple options:
1. Get a new crank (if the bike is new this is the best option. Talk to the shop)
2. Grind 4 chamfers on the back crank face where the surface interfaces with the square taper hole. This will keep the crank from bottoming out on the spindle taper and the bolt will stay tight.
Note: Do not put grease on the taper prior to installation of the crank.
Do make sure the threads of the retaining bolt have a dab of grease.
Enjoy
I agree about the crank possibly bottoming out. If this is the case, replace it. Don't mess around with grinding the faces etc.
You can pick up a nice new crank from Nashbar or ebay for less than $50.
Good luck.