Double Firing and Short Shot Problem

Hello, this is my first post on the Tippmann forum.  I figure that you guys will be able to help me since you are the Tippmann owners!

I have a used 98 Custom with Flatline Barrel, Stock, and brand new 20 oz Co2.  Whenever I go to shoot the gun, the first shot will go out okay, but the 2nd shot will either shoot 2 balls at once, or it will fire a really weak shot.  When it double fires the shots don't go very far.  The Co2 is firing correctly, but the balls will feed double or weak every 3rd ball.  Also, about every 3rd shot, the gun will no blow back the bolt and I have to re-cock it.  What is wrong?  I am going to take apart the gun and oil it but I dont know what to oil, please help.

I hear that my "sear" could be worn out, or the ball latch is messed up, but I dont know what the sear is.  Where do I oil the gun as well?  Thanks for your help!!!

Also, it could be a bottle pin valve problem. Is you tank screwed in all the way? Try another tank to see if you still have the problem. If you only have the problem with your new tank try unscrewing it just a bit to see if this cures the problem. If it does you need a diff. lenght pin valve, asa pin, or a new bottle. It depends on whether it's too long and blocking/jamming the hole or too short and needs to be longer and what way would be the easiest and cheapest way to fix it.

I agree valve bolts check them for snug but don't go crazy they only hold on by a couple of threads so just snug them and use loctite, if they are already stripped (if it's a used gun mabey this is the reason it got sold) you'll need a new valve

the valve bolts hold the valve body and powertube stable inside the body, and prevent it from wiggling around or shifting.

They are the 2 buttonhead screws you see on the right side of your gun (as viewed from the back).