A very specific idle problem...

I'm wondering if anyone has come across this specific idle problem before....

When warm when letting off the throttle completely (such as when going downhill) on my 3.9 auto at over 30/40 mph the engine revs drop to <500 rpm and the engine cuts out. It doesn't do it from cold and there is no hesitation, misfiring or any other symptoms to suggest an ignition fault. The base idle is set to ~950 rpm and the ignition timing is not a million miles out.
Now I have rebuilt the distributor recently to seal it when wading and as a last resort I'll strip it and check the air gap on the reluctor but I really don't think that's the problem as I have no other ignition issues. I've also fitted a snorkel and housed the coil in an airtight box. I know an overheating coil could cause issues but again, it doesn't feel like that's the problem - there's no high speed or high load misfiring. I also topped the gearbox fluid up so I'm wondering if perhaps it's a bit high and there's some internal drag causing the engine to stall.
Any thoughts and accompanying reasoning would be much appreciated...

Anyone?

Well, there's some kind of issue with base idle imbalance kinda thing.

Idle control is off when car is moving, so there's too much resistance to keep the engine running basically.

Seeing as there is no idle control, it all depends on set amounts of fuel and air.

Check the breathers and all associated ports and pipes.

There is a teeny weeny one in the T piece passenger side front of your plenum.

Hose goes to rocker, hose goes to throttle body and the T hose goes to plenum. This plastic T piece will be super brittle as will be the hoses themselves.

The T that goes to the plenum has a little brass restrictor in it which often blocks. The engine runs on this air bleed so if it blocks you can have issues.

Mine used to cut out on overrun when I dipped the clutch and I think it might have been this.

Also check the inlet filter and pinhole on the passenger side rear rocker cover. Small foam filter and tiny hole in rocker cover, check all is free and clear.

Report back

A little further down the road....

All the breathers are clear, surprisingly none of it is brittle, not even the t piece. Couldn't find an inlet filter?

However, I think I may have found the area and it doesn't look like it is the dizzy as I'd feared. Running on petrol is fine, doesn't even think of cutting out, but sure enough, swap over to LPG and she cuts out on throttle off overrun. Sounds to me like she's starving of fuel but I can't find anyway of increasing the fuel delivered at idle. The vapouriser is an OMVL (r 90 I think)

Any help?: http://www.landyzone.co.uk/lz/f8/disco-lpg-problem-cutting-out-177485.html

EDIT: just seen youve aready seen that thread buddy. Doh!

Ae the vaporiser units vacuum controlled or anything?

If youve got it sweet on petrol then you're half way there.

The breather filter is passenger side rear rocker cover on mine, but a 3.9 might be different. I'm not sure mate!

Cheers guys, i could really do with finding specific data on the omvl - what I think is the idle bleed screw is screwed tight in clockwise