Eventually your are going to burn out a pump. A while it wasn't uncommon for pumps to die, problem is when pumps die they can and do some serious damage to your livestock..
Are you dosing and how, are you using kalk or CA reactor? if dosing how close to the pump are you dosing and how much at any given time.
Is the gunk hard or soft?
Also what does your return int he display tank look like? penductors? Lockline, size?