Without any visual to go on, It's difficult for me to diagnose the problem. One possibility that I imagine being pretty common is that your GUI might have been designed for a fullscreen, high resolution frame, and when the crt effect enforces a low resolution your GUI elements might be falling off the page. You could try drawing something super simple in the included GUI example room, like a large circle at [0,0], and then try replicating it in your game. Other things to nail down: Does your game use 3D projections? Viewports?