Think of the meter as a pull out fuse. When installed, it allows both legs (240v) to go to the panel allowing 240v service. If something is amiss withing the meter or the socket you could have intermittent service problems. I'd suspect a loose connection internally in the meter socket or perhaps from the socket to the panel itself. It could be a main breaker problem, it could be a loose connection or even an issue with one of the panel busses. I'd suspect that the problem is right at the socket where the meter plugs in and it's either a worn clip on the meter or a loose connection. Your description is a bit confusing...half the house is not 120 and the other half 240. The house has 240 available at the panel which is actually 2 120v sides, seperated at the panel. Half the house can be on one side and half the house on the other. Where you have 240v appliances both sides are "tapped" to give you 240. When a 240 appliance, such as a water heater is activated, even though one side is dead at the meter it can backfeed and power the home. And I'm not sure I explained that satisfactorily, but I sense that you haven't grasped the concept of home wiring so it's hard to educate you.....no insult intended.