I've just had this on a 1.4 16V. As you describe, The car started and ran fine, but after 30 mins of driving and a short period of being switched off, it started running on 3 and a bit. Pulled out all the
to find that cylinder no.4 has had water in the plug well. I pulled the plug out and swapped it with no. 3. Sprayed WD40 down the wells and wiped the COP with some as well. Runs a lot better now. I can't see any source for the water, but it sounds like condensation might be involved, particularly with the sudden change in temps this week.