Having had both 748R and 749R I can tell you the problem is much the same , both mine only ever cut out at tick over , never on down shifts (I don't know what you are doing to make that happen
)
I was told by John Tayforth (after sales manager at the time) when I had the 748R that it was partly temperature related and partly because of the lighter flywheel.
Apparently all 748's are more sensetive to mixture / temperature fluctuations and don't like ticking over if the ambient temperature is under 14 degrees (quite often here in Aberdeen) as cold air is denser and affects the fuelling , this combined with the lighter flywheel on the R model results in engine stall at tickover.
The cure , adjust the potentiometers to account for low temperature fuelling fluctuations and increase idle revs slightly , problem gone
