[Gav]

I seriously doubt it is a PCM 'learning' issue, it should not be so noticable, you would have experienced it before if it was the case.
Bad or old fuel? If it is knocking it will pull timing out dropping power.
Or a blocked filter dropping pressure under load.
The plugs should last 100,000 k's so I'd not think that would be it either, but they may e grotty?

As Dave said about the only thing you should experience after powering down the PCM would be a wobbly idle. Powering it down again like you did would be the first thing I'd have tried, other than that you probably need someone to have a good look.