Mine does that too (picks up a close to a 10th after the first run of the day). I thought it might be due to the trans fluid not being up to temp, but if you're holding the trans brake for 15 seconds then you should have a LOT of heat in there.
67 Barracuda street car, 408, e85, 1.38 60', 6.44 @105.9 in the 1/8 mile, 10.19 @130.5 in the 1/4...so far....
