Nothing systemicly wrong with hi-tech servos, I'm using like 6 of them right now with no isses & at least 2 of them are 645MG's. Besides from your account of the events so far you changed the throttle servo already & both glitched. That means it is NOT the throttle servo that is the issue. Did you change the switch when you changed your radio??. Never ever run those 4 AA batter holders for your RX pack they always have issues. Get a good 5 cell rechageable pack. As mentioned above check all the battery wires for damage or loosness (this includes switch wires). You have already changed the TX, the RX & the trottle servo so it is VERY VERY unlikly to be one of those that is causeing your problem. That leaves the batteries(includes the holder & wires), the switch (& it's wires), and lastly but less likely the steering servo. It would be RARE for the steering servo to cause the throttle servo to glitch but it is possible & I have seen it. I once had a steering servo that seemed to work fine but I found that when I worked the steering the RX voltage would drop too low & the throttle whould go wacky. That would be the last thing to check in your case. For you first I would replace the rx battery pack & check the switch (& wires).