Mower Engine won't rev (Hunting?)

[Crazy Dazz]

I have a Rover mower, with a fairly standard 4-stroke engine.
It must be almost 20 years old, and hasn't worked for a while, but I thought I would give it the once over before chucking it in favour of a cheap chinese replacement from Masters.

It wasn't getting a spark, so I cleaned the plug, and rejigged the lead which was essentially busted. Put in some fuel, and voila, started with one tug (better than it has in years.)

It's always had a good rev range, but now it won't get above mid-range. Once I push the throttle beyond half-way, this "hunting" behaviour kicks in.

It will start to rev, increasing for maybe 3 seconds, then falter, falling back to previous revs, then repeat. Its very consistent.

It idles fine, and will go as high as a "very fast idle" but once I push it into the mid-range this happens.

It still cuts the grass, just a bit annoying, so thought I'd ask the collective brains trust...