I had a problem like this with bluehost after just one month of being with them. They had me locked in for a year so i’d be wasting 11 months of hosting service if I moved to another company. It was at the point where I knew exactly when the server would go down and when it would come back up.
After two weeks of complaining they switched my site to one of their newer servers and I never had the problem again. So I’d suggest just nagging them until they fix the problem. Tell them to move your site to another server.