you are probably reaching the 3.5Gb memory limit. Do you use mods? Remember, b9 aerospace, kw rocketry and ksp interstellar are very large packs and even with texture reduction, may still cause issues. But a few small ones shouldn't do much harm. If you can't live without one of those packs, try loading stock game, go to settings (graphics) and set to half resolution. That made life so much easier for me. Also, there are some packs you can replace those mods with. For ex: b9 can be replaced with LLL pack, kw with AIES aerospace and so on. Hope this helped.