Grand Cherokee splutter!

[BigM]

Here's a challenge for the online pool of knowledge

I have a Jeep Grand Cherokee 4.7 1999 with a prins multipoint LPG system and I'm ready to admit defeat at trying on my own to cure this! Also this may not even be an LPG problem as it seems to happen on petrol also, but help would be greatly appreciated....

My car is slightly misfiring when at motorway speed (70-80) and only when under load.... the misfire gets worse when going up hill or switching the air con on or having the car full of people.
I have changed the spark plugs twice (now have a cooler set in) and also changed the coils (changed 4 and no difference, so changed the other 4 and still no difference).
I have had it plugged into a Jeep diagnostic unit and no faults were shown.
If I have the car under a very small load the misfire seems to be about once every second, but if I then put my foot down it misfires loads.
Not completely sure, but seems to be better when the car is first started.

As I said earlier it seems to happen on either gas or petrol so I'm pressuming it won't be anything to do with the fuel system, but did wonder if any part of my fitted LPG system could cause this?

Many thanks in advance to anybody that can help me.

Mark

[stbb]

I have just had a similar problem on my 4l v8 discovery 2 and it was caused by lambda sensor. It was fine on light load, no errors on testbook, abd was worse on gas but still there on petrol. I spoke to a few people and it looks like sensors should be changed betweek 60 and 80 tho miles and dont alwas fail, they just get show to reacting to mixture changes. I fitted a pair of universal ones that cost £56 a pair and they have transformed the car, loads of power and miss/splutter is gone. Might we worth a try as they are cheap, but one of mine was a pait to get out.