RE: DX5e in Tam Tiger
A few things I'd try. Firstly, I'd put a block under the tank when testing. Usually a block of 2x4 wood is fine, or you can use foam rubber. The block should get the tracks up off the ground and allow the tracks to run freely without rubbing or hitting anything. I know it sounds stupid, but I've had my Leo 1a4 drive off the dining room table while troubleshooting radio problems. What a disaster that was... It happened right after my wife offered to help hold the tank too. I gruffly told her to leave the tank repair to the expert. I ended up eating my hat on that one.
Sounds as though you might have something like this you're using now (after the near disaster). Before you start to troubleshoot, make absolutely sure your main battery (i.e. the 7.2v in tank) has a full charge. When the batteries get low, it makes all sorts of weird things happen. Double check to make sure your motors are wired correctly too(I've wired them wrong once or twice). Lastly check your "Tamiya style" plugs as they tend to be crap. Make absolutely sure the wires have a good connection. I always cut my Tamiya plugs out, and install Dean's plugs.
Secondly, It's going to be your radio, the dmd, or the mf unit. I'd start with the radio. The Dx5 has a ratcheting throttle stick, and it's likely the problem. It may not be in neutral. It is likely that the problem is in your radio set up. Reprogram your DMD unit one more time and pay careful attention to where the throttle stick is for the "neutral" setting.I'd remove it from the tank, connect a battery to the rx, and then install a servo on the throttle channel. Now, make sure the throttle stick is in neutral, and turn on the radio and see if there is any action on the servo. If so, then its a radio problem and Spektrum will repair it under warranty. If not, then you'll need to check further. Go ahead and reinstall the rx into the tank.
Next, I'd look at the DMD. Try reprogramming it a few more times. If it still turns on like this, then I'd go ahead and pull one out of another tank and replace it to see if it does the same thing. (if you don't have another tank, then you'll need to borrow a buddy's or do it the hard way, and assume it's bad, and send it in to Tamiya for a replacement). When you know you've got a good one, then install and see what happens. If it doesn't fix it, then it will undoubtedly be the MF unit.
For additional hints, you might try posting this on armornuts. It's got some die hard Tamiya tankers on it.