Anybody got pointers on some good ammo to use in competition AR? I tried PMC, PMP, Winchester, and Federal. The only one that would cycle (witohut jamming every few rounds) was the Federal.

I've used Federal Gold Match 69 gr. It's an excellent round; though it is pricey.
I'm going to assume that your jamming problem is actually a feeding problem, and therefore a fault with the magazine(s) you're using.
The problems could be bent feed lips, or the spring and follower are not holding the rounds in the proper position during the rapid fire string.
Leave a post for Troy in the Magazine Forum.
He's a smart guy.

Ditto. Get some good GI 20's and see if it works ok. For comp the federal is good (the AMU uses it) and also I recommend Black Hills ammo, especially the blue box moly coated Hornady 75's. You will need to start reloading if you shoot much at 600. Good luck.