Billiefly- all good comments on the L/R balance stuff.
What I wrote about flat spinning kind of assumes pretty good L?R balance- but that isn't always a good assumption on my OWN planes, no less somebody else's!
NRAD2000:
Any chance your battery is flopping around during a loop? If the loop made it flop left to right, you could be seeing this effect as you level out the loop.
Anything reasonably heavy could do the same if it weren't held in place.
Another thing that I wanted to bring up because I did not catch it is whether you have the two servos both linked to the elevator on the top side of the horn:
http://www.rcuniverse.com/forum/fb.asp?m=9439191
The original kit suggests using a Y harness, which also requires the elevator linkages to be top and bottom, leaving significant asymmetry in the throws that you would fight.
With all DS821's and the OS 91, you are probably over 7.5 lbs. Heavier is snappier, but 7.5 shouldn't snap out as you slightly tighten a BIG loop.

I have (2) HS225 servos on the the elevator, with the linkages both on top side of the horn.
Mine is under 7 lbs and can loop around itself in maybe a 15-20 ftØ loop and not snap out.
If you are using the DX7, make sure you have the slave channel's ATV at 0% both directions. I say the slave channel, because some use CH5 (gear) others use CH7 (AUX2) but either works. You just want it (the slave) to follow the elevator channel, not add its own stuff as well. Maybe there is something happening there?
Remember to use one of the last two Mixes so that the trims come across from elevator channel, like Maynardrupp said. That's a peculiarity of the DX7.

One more thing:
My elevator counterbalance surfaces did not agree with the bulk of the elevator surface when viewed from the rear.
Many people I talked to use a couple of popsicle sticks to clamp the counterbalance surface (that part ahead of the hinges) fluch with the stab.
When I used that shortcut to set mine up, the elevator halves were way off just at neutral, adding some twist that the ailerons had to be set to counter.
So give a look from the back, at neutral and other positions to see if the two halves agree, ignoring the counterbalances. Those little counterbalances are probably less than 10% of each elevator's area, so if they are off you'd want to ignore them like I do. Its the bulk of the elevator surface that you need to set equal. The errors I'm talking about (measured at the counterbalances) are one shirt cardboard thickness up on one side, and two cardboard thicknesses down on the other, FYI. That's about 1/16" on one side, 1/8" on the other when the bulk of the elevators' surfaces are at neutral.