Firstly i would say check your power management settings, since some sort of "sleep after ... minutes" option may be enabled, there are options that can turn off all peripheral devices when power management activates.
If there's nothing there i would say uninstall your graphics drivers and any drivers that are tied in with your graphics card, get the new drivers and install.
If that doesn't work i would look at unmounting and re-mounting your graphics card physically, maybe cleaning the slot as well. if your mother board has 2 pci-e slots i would change to the other slot, also checking bios for any options relating to your GPU.
If your case is really dusty i would clean it out since your card is reaching nearly 100 degrees which is really hot! you must check your ventilation in your chassis, since I've got 2 HD4850 and they never go over 60 degrees under load.
Finally if nothing works i would suggest saving up for a new card, since newer cards produce Less heat, or have better heat management.