Are you saying that the pump that wasn't working worked ok without the Varistream?
And that the varistream was working as far as it controlled the pump speed?
If the varistream changed the pump speed, then it working.
My guess would be that it has to be air getting into your system.
The pump works when wired direct, so the pump is working, if water was gushing out, I would dismiss the dirt theory.
That only leaves airlocks, you have eliminated every thing else.