I'm a big fan of PMAGs but I've recently started trying Troy battle mags, they do equally as well IMO, not sure if they will stand up to the same kind of stress as a PMAG though. TBMs are cheaper too.
I've had the same experiences as you with mags and M16s, aluminum issue mags are notorious for bent feed lips and catches, I was issued one that would lock in enough to hold for one round and would fall out after the first shot. Go with a quality polymer mag (Troy, Magpul, a couple others, NOT ProMag) and if that doesn't fix it it's the rifle.
Sgt. of Marines, 5th Award Expert Rifle, 237/250
1st Award Expert Pistol, 382/400
D Co, 4th CEB, Engineers UP!! OEF 21JUN-20SEP2011