The first/easiest/cheapest thing to do is to remove both battery connections, wire brush them and the battery posts until they are clean and shiny. Then securely renstall them and tighten their nuts down. Test the tightness of the connector by trying, hard, with your hands to see if the connector can be rotated. If you can't move it, good. If you can still move the connector, it or the battery post is worn and you need to rework the connector so it will be tight again.
A bad battery connnection can cause strange and hard to diagnose problems with your symptoms fitting right in.
Don't make the mistake of just looking at the connectors and pronouncing them as ok.... they can look fine but not be.
If this isn't the solution, we can then try other likely problems. Do this first though.