Derailleur help
I'm totally stuck, please help. I've been trying to get this bike running smoothly. It's nothing special a GT Pantera, the derailleur is a Shimano Altus, 9-speed. I can get it indexed nicely and shifting smooth when I ride it dowhill-not applying pressure to the pedals. But the tiniest bit of resistance causes it to shift hard, skip and clunk in gears 6-9. The lower gears shift fine in any condition. I replaced the hanger, put a brand new cable in and I have indexed it. Short of replacing the derailleur entirely I'm at a loss. Any advice???