No, it is NOT normal.
Most likely at that point, you're seeing some serious vibration from the motor as it leans out and quits.
What are you using for mounting tape, and what helicopter?
I fought a 3GX this summer on a Gaui NX4 for a while, until I changed out the mounting tape.
In the original installation, (working from the heli mounting plate, goind upward) I used some thin, foam tape that came with the NX4, the mounting plate that came with the NX4, and a layer of 3M gray (#4011) exterior mounting tape.
At higher RPM, the heli would tuck its tail down under and nearly go out of control. At a somewhat lower RPM, it behaved, but I had trouble with getting the tail rotor to behave under rapid acceleration/deceleration.
I removed all tape, and again, working from the heli mounting plate and working up, used two layers of 3M gray exterior mounting tape (#4011), the metal plate, and a single layer of 3M gray mounting tape.
Problem solved, all bad habits gone, and the heli flies very well now.
I have a 3GX mounted in a 600 Nitro Pro and a 700 N, both using the 3M gray double sided tape, they fly very well, no bad habits.
If you're not currently using it, install the V3.1 software.
As an aside, how are you supplying power to the 3GX, are you using an external standard receiver, or satellite receivers?
Make sure you have GOOD, CLEAN power for the 3GX, preferably applied to both of the lower, horizontal three-pin connectors.
I currently have seven 3GX units installed in helis ranging in size from 250 up to the 700 nitro. Properly mounted and set up, and given a good power system, they are quite reliable, trouble-free, and easy to set up.
This stuff works well ---
There is also some 3M stuff with the same red backing, but the tape itself is clear and kind has a Gummy Bear feel to it. It's called 3M VHB, it works well, too.
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