Mazda Protege 1.8lt (Australian Ford Laser) equivalent

Hi all

Chasing some advice on some issues Im having with my daughters Laser. It appears to be losing spark on leads 2 + 3 when it warms up, leading to rough idling, loss of power etc. It is an intermittent problem as well.
I have swapped both the leads and coils around it runs ok for a few minutes then loses spark on those leads. By swapping I mean I have changed all connections and leads on 1-4 and connected 2-3 leads
Took it to a local mechanic, (+300 dollars later) codes said Oxygen sensor fault so I changed the sensor tonight and no change.
Am going to check for air leaks, power issues with alternator etc as well.

Is it possible just one of the coils has an intermittent fault and this could be causing the issue

Cheers from australia

Any other thoughts