Classis RR driving at speed issue?

Hi all,
I have a 3.9 petrol auto classic and when driven over 50 mph the engine constantly "misses a beat". At idle and lower speeds the engine purrs no problem, loads of power, easy starter. I was wondering could this be auto gearbox "chatter" - hopping up and down gears etc. Any ideas?
Richard

If you have a drop of engine speed when going over that treshold speed and a rise of engine speed when slowing down below that speed , then it's probably just turbine lock coming on and dropping off (gearbox starts to slip).

Yeah I thought it might be something like that except it "chatters" constantly at the same speed without accelerating/deccerating.
Richard

It's likely your engine has a misfire, and this is hidden by the torque convertor below 50mph, when it locks up above 50mph it will be noticable.

It might be something like that. When the gearbox slips the misfire is not felt so easily. So do you feel the "chatter" at some constant road or engine speed, not above or below it? And the chatter comes from engine/gearbox? (sorry about the questions, I'm not native and it takes me a couple of time to read your writings to get all the nyances hid in the sentences).

If you feel it above:
Misfire: I'd check for constant sparks first at all plugs, then timing. If the fault is not there, I'd go for fuelling issues starting with possible airleaks, then measuring CTS, TPS, lambda (if you have them) and AFM with a multimeter.

If it is at some particular speed, engine load:
Resonation somewhere, I'd check for any play in the drive train and the engine and gearbox bushing, propshaft UJ's and so on.

It seems it is due to engine misfire as it happens at all speeds above 50mph. I did change all the spark plugs but no difference. It could be my problem is due faulty leads or timing. However you mentioned a possible air leak in the fuel line and this seems more plausible because if I leave the car standing for a few days without starting it is difficult to start suggesting the fuel lines have filled with air. I will investigate further. Thanks for the advice.

After looking into ignition, check all the hoses connected to intake areas, vacuum advance unit, brake servo (unless you have ABS) etc. A crack in the hoses may give unmetered air to the engine, causing some running problems.

IF you drive at higher RPM:s with first, second and third gear, do you get the chatter then?