Some thoughts ...

Before the fuel pump module was replaced would the car do the fault regularly or could it behave itself for along time ?
If it was playing up regularly it seems to much of a coincidence to behave for 3 weeks without faulting

You say after the fuel pump module was replaced all seemed well and you have had a good few miles done to confirm that it was good ?

What if either of the In tank pumps are pulling to much current and over loading the pump control module and when the pumps cool back down this allows the module to work correctly again .

I would be scoping the heavy wires to the In tank pumps and see what they are pulling out and the current ripple may show a bad armature .

Some of the bigger mercs albeit diesels that have stirrup type fuel tanks had an issue where one side of the tank would empty itself ..... this would cause the submerged pump to over heat and burn the relay contacts .

This fault was caused ultimately because the high pressure diesel pump was at fault and not returning enough fuel to one side of the tank .

As with any module failure you have to ask why has it failed ....,is there an underlying fault ??

Does the fuel control module vary the voltage to the in tank pumps as engine demand increases ?

From a diagnostic point of view I would be looking at fuel pressure .... compare to spec then when car faulted see what's happening to the pressure at that point .

Was intermittent seemingly random cutting out which would stop car starting for an hour or so but then would start up again fine.

Did two 170 mile runs in the car with little obvious issues save fire it doing 29 mpg on the way there and just 24 on the way back despite driving it the same on the same roads. The morning after the lower mpg run it cut out.

When driving slow and steady at say 1300rpm the revs would oscillate by 1/200rpm even under constant load.

Sily point but will mention it seems to have cut out almost exclusively when turning hard right!

Wouldnt that point to the fuel not being properly transferred from one side of the tank to the other so that the empty tank is then supplying the engine with not enough fuel? :Confused:

Yup but I mentioned that to the garage and they glossed over that point so I was cautious to mention it.

Now that I think of it, when I was listening to the pumps individually the left side had both the sound of the pump (the low buzzing hum) and the sound of fuel trickling like a small waterfall. Right side had only the pump sound.

Turning hard right thing .......water in a module ?
Is comfort module on floor under pass side carpets ?

Well passenger footwell did get water ingress so possible the stuff under the carpet is wet, but would the comfort module relate to the engine cutting out?

Most of us feel uncomfortable when the engine cuts out! :p :rolleyes:

Haha! :)

When you say plenum chamber flooded with water you are on about the rear bulkhead ? is the engine ecm in there ??

Under windscreen area I was told had a lot of water in it and blocked drain holes