It's completely normal to replace the board nearly every year, NOT! I'm in a similar situation and I've decided not to fix my 22 year old tub any more and enjoy more money in my pocket and a lower electricity bill. Maybe in a few years I'll get a used one.
You'll probably never see this but I bet a lot of people (like me) would like to know how long "your" tubing lasted, since it's been almost 3 years since you posted, before you had to replace it?