[Mechvet]

Not sure what could cause erratic motor performance without digging into more detail.

To start with, have you started with elimination-style troubleshooting? For this case, it would start at the PDP and end at the motor itself. I'd personally start with a second speed controller/harness, and swap one bit of the system at a time until the problem went away.

Secondly, We found a solution to adapt a CIM to 1/2" hex shaft that might work for you. It's so far help up to fairly intense testing on two separate robots. We bored one side of the hex to 8mm, then fixtured the hex and slit it up the shaft just far enough to fit the 2mm key. The key gets put into the motor shaft, the hex shaft gets slid on, then a hex shaft collar "clamps" the split shaft together, also retaining the key stock. So far across four motors on two robots doing arguably too many test-shots, we haven't had a single mechanical failure or any adverse wear. It's not elegant, but it works.