Is it just TF2 or all Source engine games?
You might want to try updating drivers, verifying game cache, or deleting clientregistry.blob. But those are just general very non-specific troubleshooting.
The source engine can run up to DX9. Try using the dxlevel 9 (if I'm not mistaken) in-game.
I tried to change the level in-game but it wouldn't allow me to. Are there any others way that I could?
What is your rig?
More importantly, your graphics card?
Also, have you tried to use any FPS configs? Those usually switch the dxlevel to 8.