Media Pending in Project Files

Seriously frustrated! Premiere Pro keeps giving me massive issues even though I am editing at 1/4 the quality, I have deleted my entire media cache, and I have optimized the RAM performance. If I move from Assembly to Colour or Editing and then back to Assembly, premiere can not reload my project files. The project files become Grey thumbnails (similarly to when you start up premiere and it hasn't loaded yet) and when you click on them it says Media Pending forever. The only solution is to hit Save, then force quit using my task manager (because at that point if I hit close it just freezes) and then I restart the program to only get about 10mins of editing before it happens again. Premiere is not able to keep my project files loaded for some reason. The project files are almost always showing Media Pending screen. Help!! This is such a massive headache.