I once went on-site to troubleshoot an airliner with an uncommanded roll problem. The operator had rigged this and rigged that and were in the process of ordering new aileron power control units, pretty much out of desperation. I asked about the trim actuator, which they had replaced with a known good unit off another aircraft, making them think that couldn't be the problem. I centered the trim and went to look at the position of the rig pin holes in the aileron control circuit. They were mis-aligned.
The trim actuator on this airplane was an electric linear type with an internal Linear Variable Displacement Transducer (LVDT), that supplied the position signal to the cockpit trim indicator (or EICAS in this case), that was "dithering", that is, the signal was drifting around. When you thought the trim was centered, it was actually offset, and this error would randomly change as the actuator was moved. On the original airplane the actuator was installed on, the dithering wasn't enough to cause a snag to be raised by the flight crew so its internal problem went undetected.
The variations in roll you saw at speed may be some other phenomenon, or it just could have been coincidental variations in the trim indication on that flight.
So, if it was me, the trim system itself would be an item to cross off the list before proceeding to more desperate measures. Do a thorough functional test of the trim system and its indication, and also look for things like backlash within the actuator itself or in the linkage.