Driving in sill temps

I have read the post for fluctuating temp guage and i am getting cooler temp readings too but my thermostat is working fine. My problem with this weather is, when driving home at say -6 for 20 miles, there is a slight build up of frost on the front obviously. But when i get home and turn the car off to unload it then restart to put car away for the night, the revs shoot up too 2/3000 for a few seconds and when they drop the car stutters like its going to stall unless i take the revs back up then its fine. Is there something freezing behind my grills? i have the R32 bumpers and have cut the diamond shapes out of the cooler vent on drivers side. If i leave the car for say half hour before putting it away it don't do it, so it seems to me that maybe it has thawed out with the bit of engine heat there is under the bonnet, would i be right in thinking this? if so what is it?