RE: Spektrum DX-7
Richard,
You are running five heavy duty, digital servos as well as two others. This may be pulling down your battery voltage. You didn't state how many cells in your Hydramax pack. From Tower's web site it appears this pack has four cells. Is this a low impedance NiMH pack? Does the pack voltage stay up when you move all your control surfaces simultaneously after having used it for some time?
I would guess you put too much of a load on your battery pack after having flown eight minutes. The voltage dropped and the receiver re-booted. As Danny Snyder replied to a similar question, the DX7 failsafe works for signal lost, not battery failure.
Allan