I've repaired a few gimbals and taken many more apart. Some observations and some things to test... Just one man's opinion of your problem.
With the props off, arm Solo and roll the camera/gimbal by hand to check the motor's resistance from level. How quick does it level off when released? Gradual or dead slow, everything is relative. When Solo is armed, the gimbal motors are in high torque mode. Otherwise with Solo on but not armed, the gimbal motors are in low torque mode... So the test is done when armed. The Yaw motor is a good reference to how the Roll motor should feel and respond. Both have similar lever lengths.
Interesting discoveries from the disassembling of gimbals. All the motors on the gimbal have bearings to support the axle driven. The Roll motor is the closest to the ground and is the most exposed of the gimbal bearings. None of the bearings rotate beyond 270 degrees as a normal application on the gimbal. So they have little ability to clear themselves of any grime and it's friction.
I was amazed at the amount of dirt that collected on this particular shielded bearing, on both sides. On one disassembled unit the bearing was fairly tight when turned. I applied a lubricant and blew it out to flush the bearing. Night and day difference once cleaned. I suspect this could be a source for a lazy gimbal, at least how you described....got worse with time.
I don't recommend disassembly to get to this bearing. It can be fairly destructive if you haven't already "broken" down a few other gimbals. I'd try blowing it out only. Use of a lubricant is likely a bad idea, while assembled, if the lube got into the Roll motor, on the ribbon or adjoining circuit board. Your gimbal, but just my advice.
The bearing is located behind the camera holder on the Yaw arm. The inner race and a portion of the shield is exposed, the shaft adapter is protruding at the hub. Mark the bearing with a sharpie to give some reference for movement, for the inner race and the shield.
There are other causes for a lazy gimbals; electronics or another mechanical bind. So this may not be the answer to everyone's issue having similar symptoms. Again just sharing my experience for what I've found.
