My 4.6 1998 rangerover is fitted with a prins LPG system.

The tickover has been reset to the standard 625 rpm on numerous occasions by a 4 x 4 specialist.
The diagnosic readings are always correct.

After a period of time which may be 10 minutes or 10 weeks the tickover suddenly increases to around 1500 rpm

Does anybody have any idea whats causing this and how to cure it.

Your idle speed has not been set to 625rpm by your 4x4 "specialist" as the ecu controls the idle speed,the only way to change it is to change the EPROOMS in the Gems ECU.The 1500 rpm idle speed is most likely being caused by the ECU not seeing an output voltage from the throttle position sensor that is equal to or less than the stored throttle position voltage.A badly adjusted cable may cause this,if it is just make sure the cable is adjusted to give free play - if not it should have a reset done on testbook.