Replacing the radiator cap would not hurt but it probably is not the culprit in the 5 minutes you say it goes into fault. It certainly would be worth a try. If you find that it improves slightly, you may want to drain, clean and flush the cooling system and refill with fresh 50/50 propylene glycol and distilled water.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser w/ Banks 450Hp
06 Saturn Vue & 06 Chevy Z71 4x4 toads