I had a slider on a BSA that was sticking in one spot and the stanchions were perfectly straight. It turned out that the slider had suffered some impact damage at some point in its life and was dented slightly. I used a small diameter flap wheel in a drill with an extension bar to abrade away the offending area inside the slider and all was sweet after that. Let's hope yours behaves itself and wears in - or..... only go out when the weather is warm
