Nissan RB30 stalling.

Hi all! Having read Dean's question about his Disco stalling I need help with a similar problem with my 1992 3.0 litre petrol GQ Patrol.
For the last 4 months I have had many occasions where the engine starts to slow down to the point that it is doing about 10k/hr flat out. (No wisecracks please!) On stopping and with the clutch disengaged the vehicle will rev to about 2000 revs or so, full acceleration. If you turn the engine off, wait for a short while then restart generally it will come good, but not every time. The loss of power can occur while you are driving, continue for a few kms then come good while you continue to drive. It feels like loss of fuel flow but when this problem is happening the sight glass in the side of the carby's float chamber is full. To date I have replaced the fuel pump, tank filter and the in line fuel filter. The carby has been removed, checked and cleaned. Nothing was found inside the carby that may have been causing a blockage. Fuel pressure was measured as OK (but I have not done a fuel pressure check when the problem is evident as this would require me to fit a fuel guage in an accessable location and drive around for a week or two like this). I have run the vehicle with the fuel filler cap loose in case there was a vacuum problem stopping fuel flow. No luck! The problem may disappear for a week or two then happen 2 days in a row. It is intermittent. Nissan, Muller & Jacka, my 4x4 mechanic, Nissan dealers, auto electricians and a myriad of others cannot help. Can you?