I also agree. You would have to really work hard at bad reloading practices to get a 500 to 800 fps differential. I honestly don't think you are, reloading incorrectly, that is.
The first thing I would do is eliminate for good the possibility of ultrasonic gases tripping the chrono. Back up to at least 20 ft and try 3 shots. You said you can't get any further than 12' to 14', but unless the wires are tied to your rifle (
) you should be able to back up as far as you want to. It just means the control box will be a few feet in front of (and to the side of) the muzzle.
If you still get a huge variation in velocity, then you should start looking at the rounds themselves. You mentioned that you are trying to get close to the lands. For a 3 rounds test, back off a bit (shorten the OAL by .010") and see what happens.
Do you have a .22 rifle? If so, see what kind of velocity and variation you are getting from it.