What are your PC specs, esp CPU, graphics, PSU, RAM and mobo?
Zanmato identified some likely culprits. Easy way to test for overheating is to remove the case of your PC. If problem stops, then it's likely heat. Clean the ventilation ducts and fans as a first step.
If you're comfy rooting around inside your PC, disconnect and reconnect all your cables and power connections and reseat your RAM and cards. This probably won't help, as I assume the PC is not behaving erratically with any other program? However, I guess Civ stresses the CPU and graphics, so maybe...
Another possibility is some other program running in the background. Can you think of anything you had going on the bad days, but not on the good? Do the crashes happen around the same time of day--maybe some scheduled update or scan interrupts? Try turning off all other programs, especially anti-virus and any monitoring utilities, before playing.