Ok, let me take you through this step by step.
Step 1. Go to a map where this problem happens.
Step 2. Open the fps counter (Also shows the .bsp name) by typing "cl_showfps 1" in the console.
Step 3. Type "map_edite bspnamehere" into the console. I'm not sure if it matters if you type the .bsp file
Step 4. If that worked, type "buildcubemaps"
Step 5. Press F5 to make a screenshot. Upload it for us to see.
Oh btw, it will say late precache of envballs.mdl when you use impulse 81. It will still give you the weapon. It's in slot 2 I think, above smg1. I think nextweapon and prevweapon will cycle your weapons in Portal, I'm not sure. I don't have that version of the engine.