Chain dropping from 53 to 39 under load
My bike has 2010 SRAM force with red cranks, when I am in the 53 in the front and 21 or 23 in the back and really putting pressure on the cranks, the chain will drop down to the 39. I know this is cross chaining but sometimes in races I have to ride like that. Took the bike to the LBS friday, the wrench looked at it, adjusted it a little, I took it for a quick spin and it seemed ok. Then in yesterdays race the chaing started dropping again.
The FD seems to be lined up correctly, any suggestions as to what may be causing this?
Thanks