Discovery 3.5 'hunting' at over 2500 revs

HI
Just bought my discovery 3.5i V8 last night, driving home if I go over 2500 revs it starts hunting/missing the faster I go the worse it gets nuff to say if I hit nearly 3000 revs its is undrivable, however driving at 2500 or under it is perfect, has had a professional conversion to gas, does exactly the same thing (I ran on both to check), I have had various rover 3.5 in my time (mainly cars and a couple of Classic Range Rovers) but never experienced this type of issue.
Next charging light is on but is charging OK
and finally (for this 1st round)
Fuel gauge doesn't register (shows empty all the time)
Thanks in advance for any feedback/help.

Take it back to where you bought it from?

Didn't you test-drive before buying?

Sorry should have said, it's a 1991 Discovery 1, and it was bought privately (sold/bought as seen) I don't really have a problem driving at 56 mph, which is the speed that I can achieve before things start happening, I am going to change the spark plugs and the dizzy (a name I have picked up since trawling this forum) and see if that rectifies the problem!

inlet manifold gasket is commonly a reason for hunting, check charging with a voltmeter, looking for 13.5 volts or thereabouts, earth the sender unit in the tank to see if its the sender or guage.

Hunting/missing at speed could be inlet leak but if there is a problem with the alternator and/or ignition system you could have either an injection misfire or an ignition misfire.

Check alternator output, you should be seeing min.13.6v across the battery at idle - 14.4v is top output. If outside this range suspect voltage regulator. Check to see if the misfire is worse when you have electrical load on - heated rear screeen, wipers and main beam should do it. If the increased electrical load causes a more noticeable misfire the problem is your alternator. I can't remember why but I had that problem on my 3.9RRC.

If battery and alternator check out O.K. have a look at the distributor. As a precaution I would be minded to suggest fit new cap, arm, leads and plugs. make sure you fit GENUINE LUCAS arm and cap. Download Rave and get the ignition system checks. Using a digital volt meter check coil and ignition amplifier.

If none of this seems to have any effect or does not indicte what the problem is you may have to remove the distributor and check the mechanical and vacuum advance are both working bu tbefore you remove the dizzy do check your ignition timing and do mark the position of the dizzy and do remember the rotor arm will move when you pop the dizzy out ( Lots on this do a search!)

One other check/test you could try is to run the engine up then disconnect the air flow meter then go for a drive and see if it still misfires/surges or otherwise misbehaves. The injection system warning light will (should) come on when airflow meter disconnected but don't worry too much. If the engine is smooth with no airflow sensor then you'll be wanting a new one.

Hope that helps ya.