Those peristaltic pumps look very familiar - I have the same. No idea on current they take as my multimeter can do only 250 mA. They draw more than that.
You can easily take off the head, and open it up to see the tube and the three rollers. Start there, see if anything is getting stuck there or so. Sediment I expect to largely be pushed through, unless it's sharp and becomes embedded in the hose wall. Bio fouling normally doesn't happen in a mere 6 hours.
Impeller pumps probably won't work for you. There are a few major differences.
They have no fixed flow rate, it depends very much on how high the liquid has to be pumped.
Their flow is usually much more (in the tune of 3-10 litres per minute vs. 10-100 ml/min), even for tiny 5V aquarium pumps.
When you switch off the pump, the liquid will immediately start to flow back through the pump (I'm actually making use of that for a flood/drain hydroponic system: an overflow to limit the level, and the pump hose doubles as drain).