Intermittent juddering when engine gets warm.

I've got a manual '97 Micra 1L which has done 80k miles.
It ran fine for years until around a fortnight ago, when it suddenly lost power without warning and the engine stalled.

I managed to start it only once after that, after an hour or so, but it stalled after a few minutes, and that was the last time until I got it back from the garage.

After they'd worked on it, it now starts slightly quicker than before the incident, and is reliable when cool, however they did warn me it was ropey when warm.

At this point we were up to £330, and I didn't fancy spending hours more labour with them fiddling about, trying to find out the problem. I got the impression they'd rather work on newer cars.

Anyway, when I got it back, it's fine for 15 minutes of driving, but then it starts juddering/ bucking until I put my foot on the clutch, at which point it idles fine again.

I can rev it in neutral no problem, just when it's powering the car. If I let my foot off the clutch at tick over, it will drive fine and seems to recover for a while. The engine isn't getting any hotter than usual running temp.

Since then I've replaced the spark plugs (to no avail), I've also measured the (original) HT leads, they're 2k ohms for the short ones, 4k for the long. I'm thinking of replacing them anyway (is £30 a reasonable price?)

The parts the garage replaced (from the invoice): Distributor cap, rotor arm, ignition pick up.

Thanks for any help.