And everything you've been describing is leading me directly to your fps, despite denying it.
To put it as simple as possible, when a map in gmod runs at about 40-60 fps in singleplayer, add a bunch of mods and that's perp. When all of these things happen around you, some of them can cause your fps to suddenly drop to 0 for just a split second or over multiple seconds. Which causes your game to freeze. You can't look around you, there's nothing you can do until the game responds again.
This especially happens when you enter an apartment front door, because thats where area portals are. Before you enter the apartment, nothing is rendered inside, despite having the front door open. Once you begin turning that corner, things begin rendering and when you pass the areaportal (which is located exactly where the door is) everything might freeze up.
Still sure it isn't your FPS?