While I've never noted normal amounts of CG unbalance to cause what is described, I stick with my claim that the grossly forward balance Spacewalker acted as described. The problem cleared when properly balanced. I can even be more specific, the plane required four ounces of tail weight added to the 1/2 oz that had been given it during the first and evidently faulty balance.
The only other time I've experienced roll instability is fairly recently when refurbishing a club trainer. It wouldn't continue to roll over like the 'Spacewalker, but it was not steady. The cause was the trainer was set up with very hot controls and the old servos in it were simply not centering adequately for the setup that because it used L bend and keepers, had aileron linkage fully out on the servo arms to prevent binding of the keepers. Moving the linkages to the inner servo arm hole went a long way to steadying the roll.
If on the OP's plane, the aileron links are full out on the servo arm and the throw then reduced to tame the sensitivity, that would leave the centering variances to have more influence. Better would be to keep throw to around 100% and reduce sensitivity by moving linkage inward on the servo arm.