Transmission shift eratic

[Ford, what else]

Hi fellow Ford people. My 1st time on.
I have an AU3 dedicated LPG. A great car BUT the transmission has a mind of its own. I can be cruising along at 100 kph or where ever and it will back shift for no good reason, or as I slow down it will back shift. It could go for weeks without a problem and then give grief all day !!
I have been told by a transmission bloke that it was a common problem and it was the inhibitor switch (neutral/start swith). I replaced it with a brand new genuine part but its still the same. Anybody got any ideas ??
Cheers folks.