Finding pin holes in a closed loop system

[my2rotties]

Well I did all that was mentioned and then hubby did the same. The pump will work fine, then suddenly have little to no flow, and then the bubbles with low to almost no flow. It has worked fine for abut five weeks since it was installed, and now for no reason at all it seems, it is doing this... I removed it and cleaned it but there was nothing in it at all.

It was not too bad at lights on but now it hardly flows at all and I have billions of little bubbles then none... then bubbles etc... etc...

We paid $600 for this pump and will be buying a different brand sooner then I would like. Prior to this one, we had an Iwaki and the same thing happened after about a month.