Pololu has given me decent tech support . . . and they occasionally (on rare occasion) post on SoR. But I've never done a return.
It seems that one of the $40 encoded motors I bought has a defect and is internally binding. Explains why it's been so hard to get steady results during my hours of PID tuning.
I've noticed those motors on their site . . . they're cheap, but to me they look like the 'made in china' type. That type sucks in terms of efficiency/quality.
For example the Tseng Neng motors, popular because of price, but horrible in terms of efficiency/quality. And if you put a screw in too tight when mounting the motor, the gearing box will jam.
If you've only used cheap motors, you should consider giving an expensive quality motor a try - you'll be amazed at the difference. Cleaner, quieter, stronger.
btw, you didn't label the axes or even title your chart, so I can't make sense of it