Rear derailleur low limit adjustment changes under load
Before you go any further, you should check (or have it checked at your LBS) your derailleur hanger. Oftentimes when the RD acts up and won't "tune", it has a poor reference point to start from. The derailleur hanger is easily bent out of alignment,and can render the rear an unreliable little beast. 
Double-check for any side-play in the crank, to see if that isn't a contributing factor, and get that hanger looked at. Good luck.