Poulan Pro 18 hp won't start?

I have an 18 hp Poulan Pro that won't start. I have gone through 3 starters in the last few years. Engine will turn a time or two, but then gets 'stuck'. I have cleaned and checked electrical connections. It seems that the starter isn't getting enough power. I have checked the keyway on the flywheel and it appears to be good. Any help would be appreciated.