My run about seems to be holding back/lacking power recently. First of all i thought it was the cold but even when driven for an hour its still holding back when you put your foot down. My first thought is plugs, can anyone confirm this? It runs spot on, no miss fire or anything like that. Just feels sluggish and under power a certain revs and gears. Not fuel injection btw and its recently been serviced (but the plugs where not changed)

Cheers,
dan