causes for irratic or low flow readings

[chris-mr2]

Having issue's with flow being shown on my gauge, upon manual testing before and after fav gauge fills up all bars and is steady, test drive with single wot run gauge fills all bars, if I try to do a run of wot one after the other in 3 gears flow drops to 2, but after leaving it a few seconds another wot would fill all bars?

Almost as if something cant keep up, I have vented lids with a howerton engineering 2 tank set up and they are full to the brim.

Have taken evrything apart and checked for debris and purged system many times but still having issues.

Any thoughts on what else I should be checking?