Gmod Startup Problem. PLEASE HELP!

When I start Gmod, the menu appears to work fine. Until, that is, I click on any option, opening a window. The window goes behind the menu text (e.g. “singleplayer”), and it makes drop-down menus go behind the menus as well. It’s incredibly irritating and makes things really hard to do. Furthermore, when I join DarkRP, the menus flicker on and off. And in any game my mic seems to flicker on and off (not the mic itself, but the indicator image). Blood becomes thin square boxes on occasion. This is so frustrating. Please help me. Any suggestions?

EDIT: I also tried reinstalling Garry’s mod. It didn’t work.