Try an aftermarket heavy duty spring kit like the Maddmann. Both the low velocity and double shooting (two shots one right after the other?) typically can be caused by reduced spring pressure...and that can be commonly caused by a new stock that changes the amount of compression of the hammer spring.

The hole into which the spring sits is deeper in the stock than it was in the original back cap. This compresses the spring less so that the hammer does not hit the pin valve as hard as it should, thus reduced velocity. The additional problem of double shots is that with the lower release of gas besides not propelling the ball as fast as it should, doesn't produce enough blow-back to push the hammer fully into the cocked possition...so the hammer falls firing the marker a second time.

Since that second firing was with the hammer falling with even less force there is still less blow back and this time the hammer just clunks back against the valve pin and remains at rest there.

You may be able to place a BB or two into the spring pocket hole in the stock to compress the spring enough to make it work again. If you doubt that this is the problem, remove the stock and reinstall the original back cap. I am 99% sure that this is your problem. Now you not only have a probable solution, but a slightly better understanding of the marker function...you have the better understanding even if the solution isn't the correct one.

Now, if you had used the search button Search  with a subject something like "low velocity" or "reduced velocity" or possibly even "double firing" I am sure you would have found the info you needed. All of us who hang out here enjoy answering questions, but some have been answered about 1000 times. The Forums are a resource, a plethora of information, not just a chat room. Search first and then ask questions.

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Thanks for the reply.  I have ordered the maddman spring kit, so hopefully I'll have that soon.  I have yet to try the BB thing, so I'll give that a shot just as soon as I can, but I blew the o-ring on my remote, so I'll need to make a trip to the hardware store.  :(

Another weird thing I found is that the original tombstone when hooked up straight to the gun leaks with HPA, but not with CO2.  however, the one from special ops does not leak with either HPA or CO2. 

And yes, I did use search before I posted and maybe it the info about the hole depth didn't pop out at me right away.  Thanks for the help and info though.  I'll post back and let you know if your suggestions fixed it.  Any ideas about the tombstone though?