The problem is not surely caused by GSX, which is confirmed by the fact that disabling UT2 fixes it.
Simconnect is designed to accept many connections from several clients program at the same time and, GSX doesn't do many calls and, more importantly, doesn't "flood" Simconnect with requests. However, we had issues in the past with users reporting problems with UT2 and some of our sceneries, that didn't worked properly because UT2 sent too many request to Simconnect in a very short time.
However, I have UT2 installed, and never be able to see any these problems, and several of us at FSDT use UT2, and most of our customers likely do. So, it's possible it's more like a problem on your system.