P38 4.0 Misfire under load

Help Please!
Have '99 P38 4.0 (post MY99 ) which has been sound for the last 4 years but now has misfire problems under load on LPG and petrol.
Starts first time and runs/revs sweet as nut when parked but misfires and lacks power when driving (worse when the engine is hot). so far it has new Plugs (NGK plats), Leads (LR), Coil packs, MAF sensors, O2 sensors, engine coolant temp sensor and exhaust silencer (it was blowing). Re-checked the plugs and they are now sooty so it appears that ECM thinks it's cold (rad cooling fans not running either).
Anybody able to help - I'm a reasonable DIYer but getting a bit lost on the new complicated stuff.
Thanks

Not really familiar with this engine but would suggest:

Check/test alternator. Check battery. Check all earth points.

Check/test crank sensor and crank sensor wires.

Have you had an adaptive reset done on the engine ecu ?

Hi i ve had a simlar problem with my 4.6 (1999) it was the same as you describe yours so i took it in to my landrover independent garage and they found out that the airflow meter was kaput so new one installed and everything re set and now it runs better than ever yippee .
ps mines also lpg.

Before lashing cash on new bits and diagnostics, do a quick check with the old carb cleaner on all the inlet joints - spray liberally and listen for the engine to pick up. It may be nothing worse than an air leak.

I had that on my last P38. The old boys in the little garage down the road found a spilt in the inlet which they lined and ground back for next to nothing. It's sweet as a nut now.

Hi, Thanks for the reply : NO I've don't think i've had an adaptive reset done and not sure what it is. I did have to have the imoboliser reset/reload by a landrover maindaeler about 6 months ago. Is that the same thing, do i need it done and if so where (newport s.wales area) ?