As others have said, PNY are generic modules generally sold at high margins (except when on special) in big box stores. I've had bad luck with Kingston Value DDR modules too (targeted for the same market as PNY) - I've not had one here that hasn't had to be RMAd. I like Crucial, Corsair, Mushkin, Samsung and I've heard good things about OCZ, PQI, Patriot and SuperTalent (mainly that ST is decent for a budget price). Currently using Corsair that I bought used from the FS/T section here so I can have better than I would normally afford - so far, so good...
But as others have written above, make sure your modules are running at the correct timings and voltage. You may have to clear your CMOS for the correct settings to get set automatically - I recommend to always clear the CMOS when installing new memory so that none of the old memory settings get left behind. Reseating the modules in the sockets can help too.
.bh.