what may cause the idling rpm to go to 1,500 rpm and stay there? usually is 800 rpm

this is recent and normally happens after i've driven about 20 minutes uphill. i've noticed some lack of engine power as well when driving uphill

Disco I, 3,9L, gasoline

thanks

 

I would have to say it is a vacum, leak.
When it is doing it take a spray bottle, like a Windex bottle full of water and spray all around the engine, the RPM's change, look close to that area.

 

thanks Spike

at the morning when i start the engine, after some stalling it stops, then i turn it on again and stops again.. i do this about five times until finally it warms up

i'll give it a try with the spray bottle