2000 Mercury Cougar Over heats

I have a 1999 Mercury Cougar that I just bought. The guy said it over heats every once in a while, but other than that its a good reliable car. Had a great mechanic look at it and he said I had a crack in my radiator and I needed to clear my hoses and put water in it. So bought it new radiator and got the hoses cleared and new antifreeze and water put in plus new oil. The car worked great for 2 days. Than I drove it half way to work and my oil light started blinking and my batter light kept coming on. So I pulled over to see what was going on. No leaks but my cooling tank had bursted the cap off and most of the water was gone. Would you say that my head gasket is bad or a faulty wiring?