i think my 5150 is dying

Your tubes probably need changing. Look up eurotubes.com, I believe it is and he sells replacement kits or there are many others online you can buy tubes from. My tube amps (in the past) always seemed to lose a lot of highs as the tubes went. I'm no expert though, but hopefully its just tubes in your case.....
 
You more than likely need to change your tubes. I'll second Eurotubes, the high gain set sounded killer in my 5150! I'm having 5150 issues also, one of my internal fuses must have shit the bed because NONE of my power tubes power up, but I still get a good sounding signal with the preamp-out to my interface. Looks like I'll be dropping about $70.00 pretty soon . . . :erk: You defintely want to clean out all the quarter inch jacks, and you may want to take the tubes out, spray contact cleaner on the pins then work them in and out of the sockets to clean those out as well.
 
here is the goodnews......the 5150 is perfect.




the horrible news.......ITS THE MESA -cryyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy-
 
I do agree with the probability of a tube issue, but try my contact cleaner suggestions first. The 5150 series has a minor oxidation problem with the jacks, and you might get lucky and save some cash.

Edit: I know this happens to other amps too, but it seems to happen to 5150s often... Does anybody know why? Cheap parts?