I hate to say you're wrong but I have a hard time believing a 300# sand filter was too small for a 1hp pump! I run a 1hp 2speed Superpump with a 200# sand filter and have for 11 seasons now with no problem. I suspect it was plumbed and set up incorrectly for the problems you describe to have happened. A 300# sand filter is definitely overkill for a 1hp pump (and it's OK to do that).