RE: Dx8 still has problems
Since you are using AR600 and AR7610, I will assume you are using DSMX mode and that you are using the remote with the AR7610. In DSMX mode, the receiver will reconnect to the transmitter in a few milliseconds after power is restored if there is a loss of power, not 5 seconds. If you are losing signal for 5 seconds either (a) you have a poor antenna installation, or (b) you have a really big power problem.
You mention 5c 2000mAh. Are these sub-C cells, or AA format? It's a VERY important difference - high capacity AA cells are NOT for the currents you typically get in a modern model. You really need to be using a short, fat cell rather than a tall, thin one. The low voltage (assuming 4V was the lowest you saw) would also be indicative of a poor cell choice, or perhaps of inadequate wire gauge, or poor connectors or switch. I think Dick was being very generous on the 5V - I would have said 5.5V is a more realistic expectable low voltage. Please check the wire gauge, cell type, connectors, and switch. Apply Ohms law - pull a high current through them and measure the voltage on both sides.
If you fly in wind or gusts, it's not uncommon to hit "pockets" where your plane is moving relative to the ground but has little/no airspeed and thus no control. I planted a Stryker last weekend for this same reason, flying downwind into turbulence and the roller just meant I had no control even though all the surfaces were moving just fine.
I'm sorry to hear about your lost P-51. Not setting failsafe is a sure way to jinx yourself, like saying "one last flight before I go home..."
It is still possible that you have a DX8 that needs to be serviced, but I think you ought to check these other areas first.
Andy