The Novak GTB should have come with a gearing guide; if you can't find it, it's located here
(pdf file). The stampede is close to an off-road stadium truck. On that form, the recommended final drive ratio for a 5.5R is 13:1. You've got a 4.5R, which would require a taller ratio, probably around 13.5:1.
The Stampede internal ratio is around a 2.72:1, and the stock spur gear is 78 teeth, I believe (Google searching for the win!). So, to achieve a 13.5:1 final drive ratio, you'll need:
13.5/2.72 = 4.96 pinion/spur ratio
78/4.96 = 15.7 tooth pinion.
So I would recommend starting with a 16 tooth pinion. If you want more acceleration, you could go down to a 15 tooth; if you want more top end speed I would go to a 17 tooth. Be careful to monitor the temperatures though - your first time out, run for a few minutes then feel the motor. If you can keep your finger on the motor can for a second or two, you're fine; if its too hot to touch for even a second you probably need to change the gearing.
There's a nice article about gearing the Stampede here
With electrical interference, it can be a very tough call. Did the glitching start when you put the GTB/4.5R system in? If yes, then upgrading the rest of the electronics would probably help the problem. If its been a persistent glitch, from even before the GTB, then its hard to say what could be causing it without physically inspecting the vehicle. I would recommend looking into a 2.4 GHz system like the spektrum line of radios, as those are less prone to electrical interference.