I updated macOS to 10.15.5 this morning, then also updated neXt to 1.658. After about 30s the screen starts jumping and the graphics become fuzzy. It happens in full screen 3360 x 2100 as well as in a smaller window. My MacBook Pro is the 15 inch 2017 model with 16Gb ram. I downgraded to 1.654 and it works fine again.
Today I also had the problem in 1.654. It stopped after some time, and then did not reappear. I will keep an eye on it, it may be related to the macOS update.