Motor sometimes goes to three engine cylinders with no alarms. Help me

[Lolly2]

I have a hard defect to spot because it seems to be transparent to the control unit, that is not detected and reported in any way.
The engine sometimes starts to go to three cylinders and in any case not goes to four.

I'm going crazy, whit the mechanic no defect !

[Moonlighter]

You need to give more information to help us to help you.

Please give details of the engine - horsepower, model, year, 4 stroke or 2 stroke?

Also tell us what has been done already to try to fix the problem - what parts have been changed, what tests done.

I understand that language may make this difficult, but please try to give the information as best you can.