i think the shop you mentioned has a valid point in checking fuel pressure, since the engine has the problem when it is off idle, and at certain times under heavy load. it does sound like a pressure related issue.
as for them ignoring the idle valve they may not feel that is the problem but it should be diagnosed.
if it was at my shop i would recommend checking lambda and see where it is set or even if it is working, due to the idle problen it may be inop.
fuel pressure pressure tests and fuel volume tests would at least eliminate the pumps as a source of the malfunction or identify a problem there. as for the pump tests they can also look at the pump waveform and measure how many amps they are drawing.
Ray
mercedessd@cox.net