Renault megane coupe starting problem!! My megane coupe 2002, wont start, occasionnaly, and batttery is foul!

The battery is ok, no signal lights on, and the engine looks like it is ready to start, but it wont!! The engine is turning over but won't ignite...this happens occasionally, at least once a day, for the past 2 months...please, if anyone knows something...help!! :-)