usually, to go by the behaviour of the car and how it feels to drive, etc, my engine power goes something like this, all the time, even from a few seconds after turning the key when its been standing a week

...but for the past few days (and it definately "happened" rather than blossoming slowly) it's been more like this, from cold (ie standing overnight... needs a couple hours to "settle" before it comes on)

basically i have to make like my batterys flat and keep the revs up 2000-2500 if i want to be able to go anywhere - and i dont just mean accelerate hard, i mean putting foot down without it slowing down. in 3rd. on the flat... even higher if i'm pulling off, because if it bogs down on a difficult start, that's it, there's nothing left to rev it up with unless i stick the clutch right down and wait a few seconds, which makes turning right either a very hairy or a very noisy/smelly experience. even in neutral the effect is very noticable... even trying a fairly mild throttle setting the response is noticably slowed, and if i punt it... splutter splutter... then a very, verrrrry slow rev-up until it hits that magical speed and goes vrrrrrm
after 5 minutes driving or so, even if i go say 3 minutes then park up for 3 and re-start the engine that's enough, it goes away, but it feels like forever.
what might be causing this, whys it come on all of a sudden, and any ideas how i might fix it guys?
highly annoying! and a little dangerous too!
(it's like i'm driving a manual choke carb car and never pulling the knob out to richen the mix or something... except it's idling well enough, only slightly rough..)