I know this is not strictly retro but... on my downhill bike I'm having problems with the gear system, all the gears shift back and forth 1st to 9th no problem but in 4th and 5th they slip when I start to pedal on just those 2 gears and I've no idea why. I've put new outer cable on as the old was a bit battered and changed the inner as well as that was wrecked but still keeps slipping in those 2 gears. The tension on the gable is fine the high and low screws are set spot on and I've looked at the cassette and the teeth on the 4th and 5th sprocket don't look any more worn the rest. Any ideas its starting to really get on my nerves.