i have been running the stock set up for years--ya know the set up that comes with a lower parts kit. so far i have only had one that was gritty and binding. it was a CMMG kit i got from AIM.
how if fixed it -- i swapped in a colt mill-surplus m-16 hammer that had the auto hook ground off. if you have a spare trigger or hammer try "the mix and match".
i am far from an AR-15 trigger expert --but i can tell you that there are several cheap ways to get a good trigger with about 4-5lbs of pull with the stock kit.
sometimes you can shorten the legs or bend the legs on a hammer spring,
if you have some polishing stuff for a high speed tools you can polish the trigger & hammer contact areas.
the high $$ triggers do work though most of the time arn't necessary.
there was a thread a while back on a DIY trigger tune-up. hopefully some one will link this??
if you want to go with a aftermarket trigger check out CDNN.
i am such a cheap ass i just polish up the stock stuff for my own use--LOL
PS don't forget to lube the contact areas!-- don't need anything special or mall ninja for the lube, the standard mill issue stuff or even mircron 3 seems to be fine--