Gun Randomly Stops Re-Cocking Itself.

First off you don't need to unscrew your tank everytime your gun uncocks, you just have to simply pull the bolt back again into the cocked position. Second, what screw is coming out? Be more specific. And if the gun shoots fine and just uncocks itself every once in a while it is probably your sear or sear spring. Your drive spring might also be old and worn and need replacing. As of now it doesn't sound like a valve problem, a little better description of the problem would be a big help though.

Yup, that missing screw is the problem. Loose or missing valve bolts cause all sorts of havoc on 98s.

If the screw wont stay in, the valve threading is probably stripped. When you look in the hole, can you see the screw threading still in there or is it all torn up or missing?

If the threading on the valve is shot, youll need to contact Tippmann about it, most likely theyll send you a new valve.

If you just need a new screw, the specs are 10-32 threaded, 1/4" length. You might be able to find a replacement at a hardware store, but you might not. Its an odd length.

If you get a new valve, just remember to be more careful about tightening those screws down. Snug them tight, you dont need to really crank them in there. You can also pick up a small thing of red or blue loctite and put a drop on each of the screws then screw them in. That will keep the screws from wiggling free and getting lost or anything.

It looks okay to me :)