C-6 1-2 Shift inconsistencies

I am chasing a problem that I can not seem to find. My 60' times are consistent and my interval times between the 330' and the 1/8 mile are consistent but my 60' to 330' intervals are varying about 2 to 3 hundredths. It might go two or three runs and be consistent and then the next run it will slow down 2 to 3 hundredths and then be fine the next pass. It is very random. Temperature doesn't seem to effect it that I can see. I believe that the 1 to 2 shift might not be shifting quickly enough on the runs in question as I believe it is going maybe 200-300 rpms over the shift point. I have yet to confirm this as I don't have a playback tach and I must have A.D.D. while driving the car but I believe that is what I am hearing and feeling. If it ever stops raining on the weekend I have a camera mounted to video tape the tach now to prove my theory. Some more information that may or may not be important to someone. I swapped convertors and it didn't change the issue. Same results shifting at 5500 and 6000. Same results shifting by hand with a shift light and using an air shifter. Had the transmission rebuilt with the same result. No difference with the band set at 1.5 turns and 1.0 turn. Shift linkage is correct and hard to mess up with a turbo action shifter. Transmission has an aftermarket servo from J&W. It also has the wider intermediate band and the extra clutches in the clutch pack. The valvebody is aftermarket from Dynamic Transmissions with a transbrake. The car/driver combo is around 3400lbs with a 502 big block ford for power running around 6.80's at 99 mph in the 1/8th. This problem started the middle of last summer out of no where. Sorry for rambling on and on but hopefully someone can help me find this problem from all the random information I have given them. Thanks!

Have you removed the servo and checked the bore recently. I found that the grit from the clutches tends to build up in there for some reason. It could be causing the servo to not apply the band consistently. I looked at mine a few weeks ago and there was a lot grit in there and the servo wasn't acting smoothly. I cleaned it up and polished the servo piston and it seemed to help. I also had to play with the band adjustment.

The servo is free in the case bore???? The Sonnax ones are too tight . Frank Merkl's advice of machining the o'ring groove .007" worked sweet for me . The JW one you have may be fine but something to check.

I got a servo if you want to try it. Did you ever check the Pump psi? Where you going Sat. I'll be at Muncie.