(T) Intermittent power loss on 1.6 petrol engine

My 1999 Brava suffers from power loss. Sometimes the power is normal, sometimes weakened, regardless of weather or engine temperature. Spark plugs and lambda sensor have been replaced recently.

I took the car to an authorized fiat service center, they said computer shows bad knock sensor, but they didn't have a replacement on stock so I took the car home. The knock sensor seems well fastened. I disconnected the knock sensor and measured the impedance: 566 ohm, which should be ok. I left the sensor disconnected and took the car for a test drive. No improvement nor decline whatsoever, for some time the power was ok, then lost.

Also, it happens that idle revs are so erratic I feel the car shaking, but only with cold engine and rarely.

When I burn most of the fuel I'll try a higher octane fuel. Till then, what should I do or try next? I'd like to fix this myself because I'm short on money at the moment.

check the knock sensor wiring. i dont have my manuals on the computer i'm using so i cant tell you which ecu connector pins to use, but a haynes will tell you. do continuity tests on all the wires in the knock sensor circuit.

which engine do you have? (i guess 1.6)

mine sort of does that, it idles kinda funny (if i rev it and let it drop, the needle bounces for a while then settles), and if i start it from cold and leave it my car starts shuddering slowly and hissing or sucking, which slowly gets louder... but if i turn the a/c on it stops.. lol
it also seems to have its good and bad moments, but most cars do that.

its probably my air filter tho, because its not in a very good place at the moment, and doesnt get much cold air (building a custom air intake when i get a new bumper), which will hopefully sort it.

Yes, I have 1.6 petrol engine.

I don't have a Haynes manual, but I got 4 pdf documents, can't remember where I got them, but I think they are a very common use and repair guide. 2 of them are supposed to be for older bravo/brava/marea and the other 2 for newer models. Anyway, it says the knock sensor connects to pins 19 and 20 in the B part of ecu. On the picture only 2 rows of pins are shown. There is obviously something wrong here, because my ecu has 4 rows of pins as you can see on attached picture. And the wires from knock sensor are attached to the other ecu compartment (the small one), I just can't tell to which pins. Could anybody with a Haynes manual look this up for me, please?

I got the higher octane fuel now and the car still has same power problems. So perhaps it is not the knock sensor after all. What else should I check now?

have you changed the plugs and leads recently?

try testing your injectors' resistance with a multimeter.
there's loads of threads about this if you search.