chain ring problem?

[Swoop]

I have a bike with Ultegra 9 speed. When I am in the big chain ring and either my 12 or 13 cog. ...( 12-23 cassette) and put any hard pressure (accelerating) on the pedals my chain will fall off the outside of the chain ring. The chain has about 8k on it and is slightly stretched but no gear slippage. Cassette and chain ring teeth are fine. My limit screws for both derailleurs are fine. My front chain ring is slightly dinged from a wreck, but I have been riding it for about 2 months and just started this problem a couple of days ago.....As far as I guess the only things that could be fixed are a new chain and chain ring? Anyone else think of anything before I take it to my lbs? Thanks.

 

[LioNiNoiL]

When I am in the big chain ring and either my 12 or 13 cog. ...( 12-23 cassette) and put any hard pressure (accelerating) on the pedals my chain will fall off the outside of the chain ring.
...
My front chain ring is slightly dinged from a wreck, but I have been riding it for about 2 months and just started this problem a couple of days ago.....As far as I guess the only things that could be fixed are a new chain and chain ring?

The big chainring is bent. Even a nearly-imperceptible bend will cause exactly what you describe. You could try removing and straightening it [that's what I did last year] but they're reasonably cheap to replace [unless you need to have the Latest-n-Greatest™].