I expect it's the friction between gear teeth and not bushing friction. My 2 large sp2 clocks are both stopping randomly now after months of reliable use. Both use silk pla.
I have an old sp1 clock that used a different silk pla (can't remember the brand) and its still running fine. I did use dry lube on all gear faces.
Going to pull both my sp2 clocks down soon and do the same.
Hope this helps.