Rough Idle

[VEEMAN]

I have a transit mk6 300 2.3L petrol/lpg on a 04 plate.
I recently had a water leak from around the water temperature sensor on the intake manifold. On further investigation, the plastic housing had a crack in it around the brass insert . To cut a long story short I purchased a good inlet manifold and fitted it. The van was working fine on petrol and gas prior to fitting this second hand manifold.
When I finished fitting the inlet manifold and started the van, it idled like a bag of hammers and rocked a lot.
I then took a look at all the vacuum pipes for splits, not connected ect. All these where fine. I also had a look at the idle bypass valve and it was clean as a whistle. I also cleaned out the throttle body housing as well when I was on.
The engine idled rough again, but once the accelerator was pressed the van reved up sweetly through the rev range. But once the engine revs dropped back to under 1000 rpm the hunting ect happened again.
On the gas side once switched over the revs shot up to 2000 rpm and the engine was smooth as well. The engine revs then dropped to 1800 rpm as the gas rpm filter cut in. the engine ran well at the higher revs.
I then stuck the code reader on on it's live function setting. I noticed that the temperature was going up, so the water temperature was fine. I did also notice I had about 0.01% on the air intake reading. Also there was no output from the lambda probes again reading about 0.01mv.
So does these above readings indicate an damaged ecu.
Can any one point me in some direction to check something I've overlooked to sort this rough idling out.
Any help is appreciated.
Veeman