My seatpost on my single speed keeps sliding down while I am riding and its pissing me off. The original seatpost was way to short and I had to opt for a new one that was one size to big because the tightening thing on the frame was stripped and beyond repair. Now the seatpost has taken a liking to moving back and forth while I pedal and slowly move down. I have tried the cutup pop can method but it always seems to work in the first few miles them fails. Anyone else got any ideas or proven fixes for this. The frame is a really old raleigh so I doubt I can find the right components to fix it.