Some parts of the map become invisible or transparent. 

This causes you to fall through the map in a never ending fall.

The land also turns different textures and colors. 

 

What should  I do? 

It seems to be occurring on highways and out of Los Santos city

to recommend anything We are going to need specs please. It sounds like your CPU, and or GPU can't run the game. Usually texture loss is cause by one of them (or both) getting backed up while you are driving fast down a road and it needs to catch up so it deletes everything and reloads it slowly. High poly vehicle models will do the same also if there are a lot of them loaded into the area. If you have replaced all civilian vehicles with real life counter parts, and all cars were 100k poly's+. You drive into the busiest part, there might be 20 - 30 cars there. that is a lot on your cpu and graphics card. I just typed a lot, but that will MORE THAN LIKELY be you problem no matter what. also, turn down graphics and see, or remove any graphics mods.

It's not my CPU or GPU.

It has worked perfectly fine in the past with the same mods in game.

Plus, the entire city with large buildings can load but the middle of no where doesn't load at all.

I have no real life vehicles besides police vehicles.

 

MY specs if you are wondering

CPU-i7 2600

GPU-gtx zotac 1050ti 4gb mini

system ram- 16gb ddr3