I thought that the water may drain towards the level in the tank. Whatever is giving me my mild flow fluctuations may be cured soon as i'm going to leave the di out of the van system completely. My pump is as low as I can get it. To me it seems odd to try and lift water up to the top of a di vessel anyway especially if sucking as mine is instead of pushing.
I ran the system flat out the other night untill no air bubbles at all were passing through - I could then turn down the varistream from it's normal 6 to 8 to its lowest setting and still pump enough to clean.
Reverse the flow & push through your di dave, I think this is what's giving you trouble. My pump is 2 feet higher than my tank, DI's same level as tank so DI heads are higher than water level & more so as tank drains!
Your pump is sucking & then trying to force water through a mass of 6mm bore hose!! IMO it's all wrong.
Try it mate, but first take top off DI & fill back to top with water if need to. You'll have to run pump flat out for a bit to get all the air out.
I run my pump on full for a few seconds before every job as the water in the pipe does drain back down after a while, but that's it. Turn it down to setting 1 & I'm away with no probs at all.