My last power loss problems (limp mode in high gears over 3000rpm) turned out to be the CAT blocking up.
Until i had taken it off (and smashed it out) i did not realise how fine the matrix was.
If you do not make regular long journeys, the engine/exhaust does not get up to operating temp's, and an older CAT can start to restrict the exhaust flow.
This was throwing manifold pressure 'intermittent', and MAP fault codes.
I run without a CAT now, it is not required in the UK, and has improved my engine.