Problem when starting cold

[Drew123]

Just recently when i have started the car it will idle normally, but if i rev it and have it sit at about 1500-2000 in a few seconds the revs will start to drop even if i put more acceleration it sounds like its going to stall the more i press on it, if i take my foot off it will go back to idleing fine, this is only in the blue (i think) area, after a few minutes its fine, any ideas as to what this could be ?. it has only just started to happen also.

Thanks in advance.