Have same problem with my 1991 rev 1 n/a.

Replaced all the plugs, leads, dizzy cap, rotor, coil after a night of heavy rain caused car to die.

HT leads lit up like Frankenstiens laboratory.

on damp mornings it starts but still dies at 2000

- 3000 on load

( high gear or hill).

But is not so pronounced.

So obviously problem still in there somewhere.

After a short stop.

the problem goes away.

Its fine on sunny dry days.

I was told when my MR2 MK1 did this that moisture builds up in the spark plug tubes and it arcs around in there.

Hence when car sits the heat evaporates all moisture in there.

Try spraying WD40 stuff into tubes

(under the plug lead caps

) to see if that makes any difference before setting off.

Did it with my MK2 when the b1tch would not start after sitting for 2 days in damp.

This seemed to make a difference.

Just a thought.

As its damp

( humidity

) related I am tending to diagnose on an electrical basis.

Odd thing is it clears when I floor pedal

?