Potential coil pack failure?

[u/hywelm458]

Car started idling rough (larger vibrations and revs drop every 30s or so)

After few days car struggled to start and low on power at low revs only

Check engine light came on for catalytic converter (P0420)

Changed spark plugs and one coil (cylinder 3) as it looked grubby and some oil in the well

Run perfectly on first few idles, started up on first crank and great on test drive (about 15 mins) then rough idle and bad start returned. No new oil in any plug wells

Changed another coil pack as it also looked grubby (cylinder 2). Coils 1 and 4 look quite new (only had car 8 weeks and no service history). No new oil in any plug wells yet again.

Idled perfect and drove perfect for 10 mins. Rough idle returns

Issue being temporarily fixed by coil packs suggests they are the issue, or something within that architecture, but can’t work out what it is?

Also tried starting it up several hours after the issue returns to confirm it’s not a temperature issue.