Large pressure drop at node in model

Greetings,

I am modeling a primary irrigation network in WaterGEMS, whereby a limited number of nodes supplying green areas are open for one hour, then closed and a further group of nodes of around the same total demand is open for the same period and so on, for a total irrigation schedule of 10 hours per day.

The project site consists of a hilly area in its middle, with the level difference at the top being around 10-12 meters higher than the surrounding low areas.  The pumping station supplying the entire system is at the lower level of the site where the supply line is located.  The pump used to supply the model is a VSP with a constant target HGL. 

While  the pressure variations throughout the network are acceptable and do not pose much concern, it is at the top of the hill that I am getting problems:  The minimum required pressure to operate irrigation emitters (sprinklers, bubblers, etc) taking into account headlosses is around 4- 4.2 bars, so I have set my pump head at 6 bars to attain this pressure.  This is happening, but when the time comes to activate the nodes demand at the high area, I am getting a sharp drop in the pressure at the time the node is active, with the drop exceeding 4 bars.  This is obviously getting below the pressure required for proper irrigation supply. 

I have tried and checked everything to make sure there was no choking point such as very small diameters somewhere, but it doesn't look there was any.

I would appreciate any help to resolve this matter.  I am including as an attachment, the pressure graph of a node in the hilly area that shows the pressure drop.

Thanks and best regards,

Amine Salameh