Help if you can! 2005 XTerra, Automatic, 56k miles. All maintenance done as recommended. Two problems - maybe related (or not)....

For about the last 15k miles, the transmission has a weird deal where it shifts into a lower gear going downhill. So if you're coasting down a hill at 60mph, around 1500rpm, pretty soon you'll be going 60 but at 3300rpm. VDC makes no difference on or off. Even after it levels out, a quick "kick" of the accelerator is required to make it upshift again.

For about the last 5k miles, the engine knocks like bad gas, improper timing, etc... when under load between 1500 and 2000 rpm. In park, revving the engine, no noise.

These problems seem to occur only when the vehicle is cold or completely warm. There's a period in the middle while it's warming up that it runs great. Dealer can't locate problem, car's been in for service 3 times with no results

I love the X, it's paid for, and this is driving me nuts. Want to get it taken care of while it's still under warranty, and hoped someone could point me in the right direction so I can point the dealer there, too.

Thanks in advance!

I guess everyone here is stumped.

Part of the answer: it turns out all the noise was being caused by the Power Valve. Shifting is still TBD. I'll post the answer here if determined.

One last thing as follow-up: the power valve replacement also seems to have fixed the shifting problem.

I also noticed that the X runs a lot better in general - more power, better acceleration, smoother overall.

I have no clue what the power valve does exactly, but evidently if it goes sideways it causes a lot of bad things to happen.