The main ground connects to the engine and would be the first place to start. I don't know exactly how many grounds your car actually has but this is the most important one. I suspect either the grounding point is rusted or the wire itself is bad. The other issue is the connections to the battery. They too can get corroded and cause issues. They are easy to clean. Try these and if it doesn't help post again and we'll see what we can do to help you out.