It seems you have some high-res texture mods installed. I have installed a streamfix to overcome this. By default, GTA San Andreas only loads a certain amount of graphics into memory due to the computers that were being used at the time of release. The streamfix allows people to use multiple high-texture mods at the same time by raising the limit of graphics being loaded into your memory, allowing for smoother play and makes sure that the other textures don't disappear. If you need it, drop me a PM.