"Come on then? Why do you think we are so poor one week. And ok for a while, then poor again? What's your view on why this is happening? "

Champs - see my reply to Seventwo. I have no simple explanation. If there was one, don't you think EVERY manager would apply it EVERY week?!
I keep saying this: football is complex. All of the following (and more!) are factors in every game:
The managers. (BOTH of them, in each match).
The match-officials.
Injuries.
Form.
Confidence.
Conditions.
Formations.
The intentions of the other team.
Talent.
The way a team "clicks" on the day.
The fact that the opposition make-up suits your team some days, but not others.
The luck of the bounce and the break of the loose ball.
Etc.

Once the game is underway, many of those factors are beyond managerial control. Also, I think that fans often talk as if the opposition isn't there at all! In reality, of course, both managers and both teams are trying to achieve the same thing.

None of that excuses the manager's overall responsibility. Carrying the can is part of what the role entails, and everybody knows that. I just happen to believe that the identity of the man in charge actually matters far less than the media and most fans argue.
No-one here or on other sites has EVER given me a convincing explanation of how there CAN be any such thing as a generically "good" manager when the same man can appear brilliant with one club, but be a dismal failure with another. (Or sometimes, have drastically different seasons with the same club!).
As far as I can see, you - personally - seem to be saying, "Let's just have a change because it's time for one."
Fair enough - it's as good as any other reason, but please don't try to pretend there is convincing football logic behind it.