I would replace the pump seals before dropping 200 bucks+ on a new one unless there are other things that you know for sure are wrong.
I'm wondering if your inpeller stops spinning (slipping on shaft) when you put the vac to it and the resistence increases....
As for the sand - how long have you had your new filter running, supposedly, they do tend to push sand out until it all compacts nicely, I'd just vac it back in for a while see if it stops. Oops - forgot, you can't vacuum