My car is so strange?

My peugot 206 diesel is very sluggish when i put my foot down. 1st gear 5mph, 2nd 10mph etc. Sometimes when i start it up the revs take time to kick in even if i have my foot flat down on the accellerator. If i have to drive for more than 20 mins it then sorts itself out. I did drive in heavy rain a while ago but checked the air filter and it was dry. I'm a complete 'engineaphobe'. Any help would be great. thx, Jel