My pilot in my furnace lights and starys lite for a number of days. Then the pilot goes out, usually in the middle of the night. I have changed the thermocouple but still getting the same results.

The pilot looks OK. Blue flame about 1 inch long with a slight yellow in the tip. Any suggestions, also should the flame completely engulf the thermocouple? Can you suggest a replacement valve, it is 30 years old.