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???

Did you align the new hanger? Is any part of the drivetrain new and the rest old? Sounds like a new chain on an old cassette OR the hanger needs aligning. Just because it's new doesn't mean it's straight, the frame can be slightly less than perfect. You have to align every hanger to the frame it's on.

Thw drivetrain is all the original parts so must be the derailleur hanger. I will try to align it and see where that gets me

...this is reasonable advice. The cogs that wear most rapidly are the smaller ones. Start with the cheapest solution first (checking alignment), then proceed upward in investment by replacing the cassette cluster and the chain. I guess you might have some mechanical problem with the rear derailleur, but that's rare as opposed to simple wear and tear or misalignment as the cause of something like you describe.

...they wear out. As they wear, the shifting sufffers.

This. In 2012 I bought a brand new bike with Ultegra 6700 that had the same problem as the OP. It took me 1000 miles before I realized the hanger was bent inward a tiny amount, maybe a couple of millimeters. Straightened it with a crescent wrench and it was perfect.

Before you start replacing parts, check your cable attachment at the derailleur. Clamping the cable on the wrong side of the anchor bolt can screw up the indexing slightly, which will then result in the behaviour you describe.

I'm really curious why the hanger was replaced. That's not a part that typically 'wears' and is usually replaced due to some kind of incident (crash, etc). Seems like something's not aligned.

Could also be that the cassette and chain are worn out.

Not applying pressure while shifting is wrong too, IMO. To shift properly you need a certain amount of pressure. Not too much, not too little. The cautions we've made for ages about not pedaling hard when shifting seem to have gotten rewritten over the many years to "be delicate with pedaling when you shift". Which isn't correct. You can actually pedal fairly normal if you are not grinding out a climb at 30 rpm trying to get out of cogs you should have shifted out of well before that time.

However, for the issue the OP has, I'd investigate hanger alignment or check that the DR isn't bent itself. This is the existing DR wasn't it?

What happened to make you replace the hanger?.

Lots of could be solutions. I know it's hard to get into shops, but paying for a derailleur adjustment will get a trained eye on your problem.

Follow-up
 

Took me sometime to work on it but I did finally get this bike running smoothly. Adjusted the hanger, did not fix the problem. Replaced the cassette and chain 👍 all is well

Thank you for all of your suggestions and help!!