Strange engine/gearbox/revving phenomenon

[Geezer]

I've got a bit of an odd engine/gearbox/revving issue that I'm struggling to put into words  it here goes....

When I'm driving at a constant speed, usually on the motorway I get this  very slight 'blip' which feels like a very slight hesitation in the engine/gearbox before re-engaging. I've got an auto, I could also describe it as a slight dip of the clutch, or as if the auto box is thinking about changing gear for a split second but hen changes its mind.

It's is most prominent when cruising  or accelerating gently between 40 and 70, below 3000rpm. At lower speeds it either doesn't occur or isn't noticeable., same with higher speeds. Similarly it either doesn't happen or isn't noticeable under hard acceleration.

What is really bizarre is if I engage cruise control it doesn't happen.

I have a 2003 4.6is with 96k on the clock that I have LPG converted. The issue is present when running on both petrol or LPG.

I was wondering whether it was a bad remap from a previous owner or if the air/fuel mix isn't right?

[JimmySpeed]

Did it happen before the conversion? If not, how long after the conversion did it start?

[Geezer]

To be honest I noticed it just after i pirchased the car, before the conversion. However, I had the whole car including the gearbox checked out by an Indy called Noel near Solihull before I got the conversion done and he said everything was fine so I just ignored it. Now it's just irritating more than anything when I'm cruising on the motorway.

[JimmySpeed]

Noel is a top mechanic. Did he drive it?

[Geezer]

Yeah he did, but not sure if he took it for a long cruise, as I left it with him. Maybe i'll take it back to him.

[IanP]

I think it could be the torque convertor

[Geezer]

Thanks Ian, at least that gives me something to search for, I was at a loss to what it could be