I am getting a really weak sound on some higher frets. I can hold a "C" note out on the 8th fret of the first string and it sounds fine. Same thing with the 13th fret on the second string. When I hit a "C" on the 17th fret of the 3rd string, it sounds terrible. Super weak, breaks up. Pure garbage. It is not just this fret, it tends to be higher frets in general, mostly on the G string. Some other strings are a little questionable, but none as bad as G string. High e string sounds great, good sustain, strong sounding notes.

I am playing on an Ibanez RG350DX, stock pickups. Just put fresh strings on. Playing through a Peavey 6505+ 112 that I just got used. Not really sure what to suspect. I don't have another guitar to plug into the amp to compare. Thanks for any suggestions.
guitar most likely needs a good setup. frets may need a grind and polish to level them off.