My Defender surges quite badly when pushed hard (uphill) or with aircon on. Always starts at 2800 to 2900 revs in all gears.Can sometimes drive through it and it settles at higher revs but sometimes have to throttle back. Has been going on for 10 years (no shop can solve it!) and is not getting worse.
Any ideas, things to rule out -- fuel supply, electrical, timing,ignition module?

As an aside what is safe revs for this engine?

Thanks for any advice.