GU 2.8 TDi Idle RPM issue

My brother-in-law has a 1998 GU 2.8TDi. He called for advice and I am unable to explain his issue. After starting the idle RPM settles on 1000 RPM. With no accessories running (aircon etc) immediately the gear lever is selected to any ratio ( 5 Speed Manual ) the RPM decreases to around 750 RPM. As soon as the gear lever is moved to neutral, the RPM increases again to 1000 RPM. Depressing the clutch alone makes no change to RPM, only selecting any gear. If anyone has an answer I would appreciate being enlightened. The only engine management modification I am aware of in his vehicle is a performance chip fitted by Berrima Diesel. I cannot see how this could be influencing the problem cited above.

Turn the "heat " switch off, r/h side of dash.
Cheers, the ferret.