As is the situation with all problems, you need to go to the beginning and work forward to your pump.
If you use a seperate battery, is that fully charged? Is there a good connection between the battery and your cable - crocodile clips make for easy connection but they don't always make good connections. Check the cable for damage and then check the fuse and the fuse holder very carefully.
Can you boost you current battery by coupling it to another battery via jumper cables? Whilst its all connected up, does the pump run ok?
The Varistream is designed to replace the pressure switch on the bottom of the pump but some don't know this. Corroded contacts in the pressure switch can create all sorts of problems as my mate found out.
You say the system has worked perfectly so has anything changed at a similar time to when the problem started?
We found that the MK1 digital Varistreams were great. The current ones that also incorporate a battery condition feature drain the battery even when switched off. A new 85 amp battery fully charged was only 75% charged after a week of lying idle. Since we have installed an isolator switch, the van's alternator keeps that leisure battery fairly fully charged now, which it didn't before.
The earlier analogue Varistreams with the dial on the front did produce similar problems you describe. We found that the pressure adjustment on the blue pressure adjuster pot needs resetting from time to time. We unscrew the 4 screws and take off the front cover. On the back of this cover is a blue coloured adjuster. Remember where it is set, and then carefully with a small flat screwdriver turn that screw clockwise and then anticlockwise about 20 times. This is an old electronic trick that helps to reseat any connector tracks that have become corroded. Return the adjuster to its original setting, test and assemble.
If you have a split charge relay, is this working? With the engine running a volt meter across the terminals should read between 13.5 and 14.2 volts. If not, suspect fuse.
If your system is connected to the van's battery, does the pump run fine when the engine's running?
Sorry, but fault identification is a methodical process of elimination. Most likely either battery or poor connection somewhere IMHO.