My guess is that it is running too hot and that the machine is getting an air block from the steam in the HX which means the pump can never build up the pressure it should. *Im no expert though so that would be my guess. *Ive found that if I let my water resevoir run dry it will let a small amount of air into the circuit and unless I bleed that out the pump wont get to full pressure.
Try giving the machine a really long cooling flush before pulling a shot. *See if that makes any difference. *My guess is that you need a new pressure stat(or perhaps just an adjustment). *If that is the case I would be reluctant to run the machine much as it would be running way too hot.