renault megane p reg starting problem?

my renault megane 1996 1.4 kept juddering while driving my husband noticed one of the earth wires from the battery was decayed so we put new ones on and it has stopped juddering but sometimes it wont start. it starts fine sometimes then other times it wont fire up, its fine when its been jumped, and when you turn the key lights/cd player all come on fine but the engine just wont start? can anyone help please going on holiday next week x