1999 Nissan Altma
If it is running hot more than likely head gasket not valve cover gasket.
Have you checked where the leak is,may be hose or water pump,radiator.
Your head gasket may have failed, or the head is cracked. The valve cover sits atop the head and will in no way cause overheating. The engine starts because the leak is not horrible YET. If you keep trying to drive the car, you will ruin the engine.
Remove the oil filler cap and see if it has a white gunk on it. If it does, you have a blown head gasket.
Looking for a Used Altima in your area?
CarGurus has 82,298 nationwide Altima listings starting at $1,900.
Search Nissan Altima Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Nissan Altima Experts