Hi guys and my apologies if this may have been covered. I done as many searches as I could had they seemed to no avail so here is my last resort.

I have a 1995 EF XR6 Dual Fuel. It starts first time, every time and runs and idles perfectly on gas. As soon as I switch it to petrol however, the idle becomes very rough.
This is a bit hard to explain because it seems like a long miss or surge if you preffer. Basicaly the car idles for 1 second then stalls for one second, idles for 1 second and stalls for one second up and down from 650rpm to 500. Imagine it like a wavy line being a bit like this: ____/----\____/----\____
I know the above may look funny but please try and see it from someone's point of view who is basicaly absolutely clueless as to what this may be. If it done it both on gas and petrol it may be a bit easier but the fact that it is only doing it on petrol has be completely stumped.

As far as take off while it is doing that on petrol however, as soon as I give it the gas, there is absolutely no hesitation, the car takes off and drives just the way it should every single time. Power wise it is in incredible state and I've been commended on that and it does not miss on high RPM or any other speeds. I seldom drive the car fast or hard but I've tried everything just to find a problem elsewhere and I have nothing until it drops down to idle. Also there is no warning of this while the car is slowing down. I slow down and come to a complete stop and RPM is still good. It could take a couple of seconds even up to 10 to 15 seconds for this to begin, however it will do it every single time regardless if the engine is hot or cold.
As far as things under the bonnet go, leads are fine, spark plugs are fine and brand new, oxy sensor is brand new.
I've read about fault codes but I do not know how to check them so here I am asking for any help anyone could give on this. My last resort is to take it to a FORD dealer to have a look at it but the problem is I am going overseas in a couple of months and the cars needs to go. My problem is I don't want to pay someone a thousand dollars only to find the problem unless I really have to and since the car has to go, there is no way I wish to sell it with problems. I have loved this car since day one and the condition would satisfy the most meticulous buyer inside and out, therefore I want to sell it knowing that it will be running as good as it looks. If anyone in Melbourne has seen a Dark Grey (gunmetal) EF XR6 with MHTSOS plates, you will know what I mean when I say it's in perfect condition.

Anyway, thank you all kindly for reading my incredibly long post and my thanks again for any kind of information you may be able to give, useful or not.

Try pulling the vacuum hose of the FPR to raise the fuel pressure in the injector rail, see if that improves the idle. If so your injectors are blocked up.

Also replace the fuel filter............

Also check for a vacuum leak to the MAP Sensor.

BenJ