Thanks for the input, I've already spoken to Tornado Alley and my own maintenance co. and they've never come across this before hence the question. What I didn't mention was that this has only started happening after its annual, there were some mild changes made to the fuel flow on the mechanical pump which is the nearest thing to the problem one can imagine, yet no fathomable reason seems to fit. The flow turbulence through the flow transducer sounds like a good idea as does the voltage issue but when the flow drops as the pump is turned on the engine demonstrates the usual effects of too lean a mixture so flow IS being reduced, it isn't just a gauge issue. The electric pump is the first thing the fuel meets after the tank selector, then it's the mechanical pump, all unmetered at this point. Have considerered the pump being faulty in some way but not sure how? Perhaps I shoud just swap it and see what happens.
This one is out there so any more thoughts still welcome.
Cheers
Steve
IO540 The low boost pump is left on for 30 mins to avoid any vapour probems in case of warm fuel, some just leave it on all the time. There is also a high boost setting for use above 18000ft for the same reason.