Question for technical gurus

Weekend before last I finally got round to welding the floor on Bertie (the Punto Cabrio). I disconnected the battery while welding and reconnected it the following day. While the car was raised I thought I would check the exhaust for leaks, but when I started it seemed to be only running on 3 cylinders. I thought this might be due to damp weather, so left it while I put the interior back. When I started it to move off the ramps it appeared to be only running on one cylinder and unable to move under its own power. I left it a couple of days to make sure it was entirely dry, but it would still not run. I had a spark, a pulse at the injectors and the fuel pump was working. Just to check, I replaced the ECU, coil and crank sensor, but it made no difference (I have since put originals back). It would try to fire, but only seem to catch on one cylinder. I checked the plugs and they seemed a bit sooty.
Couple of days later, we went to look at a new car and when we got home I tried him again. This time he started and seemed to run on 3 cylinders, then all 4. I gave him a good run and he seems ok. On Friday I drove him from Suffolk to Lincolnshire and back and he drove like a train, pulling smoothly from 60 in 5th on hills (A1 at Stamford), leaving the lad in the Fiesta on my boot in the distance. The sparkplugs are now a nice biscuit colour.
Could it have been caused by a blockage in the exhaust? I think the back box is disintegrating internally as it rattling, despite being securely mounted.
All sensible suggestions welcomed and silly suggestions appreciated!
 
Could it have been the ECU 're-learning' it's settings. Not experienced it myself but I've heard of something similar.
 
I don't think so as I tried my spare ECU which I have used before and simply plugs in and works. It did exactly the same thing.
 
Quite probably. Problem is that Liz needs to be able to rely on Bertie, so the search is on for something small and funky (not a BMW Mini). Bertie will be retained though to live a life of luxury, being pampered (I've already done the welding), used in the sunshine and tucked away in the garage when it gets cold and wet. He may burp to his heart's content! ;)
 
Something happened to the O2 sensor in the exhaust? Sounds like there might be a load of rust flakes in there, or a bad connection or bad earth braid.
 
We like the Ka, particularly the StreetKa, but we are looking for something a bit newer and the new Ka is boring. Tried a Twingo, but it was gutless, so currently looking at Fiat 500s. Interestingly, the 500 has less bhp than the Twingo, but it's far happier to rev freely and has more go about it.
 
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