There is a problem seen on some 1.1 Pandas involving the ECU wiring. It is related to insufficient support for the wiring as it leaves the ECU.
One fix which has worked for some is to support the wiring bundles in some way, without disturbing them. (Not sure this is actually a fix, rather more of a temporary get-round, but apparently it can last for years).
On the ones I have seen, there is a break in one of the wires, which causes intermittent contact, leading to a misfire and MIL on dash.
Quite easy to locate, and make a good repair, as long as you have some basic DIY electrical skills. If not, an auto electrician should be able to fix it in minutes, especially if pointed to the ECU looms as the likely culprit.
I've got one with these symptoms coming in next week, so hopefully I can put some pictures up showing the problem.