I sprayed the inside of the MAF sensor and while I was at it, I sprayed the electrical connections of the MAF, throttle and denso sensors. I looked up that a brake cleaner is not recommended because it can damage plastic, however WD40 was told to be safe -- which means there is no need to buy a expensive can of MAF cleaner from Ebay.
I went for a ride and it occurred to me that I can easily get to 60mph in third, revving up to 4500 (previously getting to 4000 was possible but very sloooowly).. it was easy to get up to speed on the motorway, the sounds of the engine has also changed, now I have a bit of a 'racing' feeling.
All in all I am very happy, now I can safely go on longer journeys. Thank you ever so much guys for all the advices; especially to busman and lozz who persuaded me looking into the MAF -- despite the mechanic claimed that they have checked it already! I am now feeling like a mechanic

Another case closed, where can I send the beers?
