Prelude 2.2 VTEC 5SP Manual JDM 1992 100,000 miles

When the engine is warming up it goes fine throughout the whole rev range.

As soon as the engine is fully warmed up and cruising then a slight touch of the accelerator will cause a loud judder to appear about 4 times a second.The problem gets louder going uphill.

If I press the clutch and rev the engine then it won't vibrate.
Once I let the clutch out again and press the accelerator again then the noise will re-appear.

If I change gear from 5-4 and 4-3 it will vibrate in those gears as well so it's RPM independant.

I've turned the ignition on and off whilst driving but it re-appears immediately.

If I slow the car down to 10-20 MPH and then speed up the engine will be ok up to and including the VTEC part. But the problem will re-appear once I start cruising again a few minutes later.

I have changed the spark plugs, air filter and cleaned the EGR valve. I have also added injector cleaner. All vacuum hoses seem ok. I've also disconnected the battery to reset the ECM. Checked there is no misfire at idle etc. Now thinking of battering it with a hammer.

I don't think it's a problem with leads or injectors because it does seem to go to VTEC at all other times.
I'm not sure whether this is related but when I rev the engine and go back to idle the engine has a slight vibration noise for a second or two and then it dissappears. This is not the same noise as the main vibration though.

There are no fault codes present so it's a pain trying to diagnose the problem. It's also hard to diagnose a problem which only appears whilst driving.

If I drive 40-50 miles the vibration will come and go i.e. it's not here for the entire journey

Please help. I'm British, so sorry if my language is slightly different.
I'm taking it to a garage next week but I would like to point them in the right direction to limit damage to my cobweb covered wallet.

Many thanks in advance

Dave