W reg Audi 80 2.6 Auto Cabriolet ..Engine misfire

Hi,
Not sure if this is in the right sub forum but I think it is. A while ago I bought a W reg audi 80 2.6 auto cabriolet which was running well with no issues. Lately,have not used it much and have noticed that I get an occasional misfire on start up so much so that last week the misfire managed to completely destroy the air filter casing.
I replaced that and at the same time cleaned the throttle body as it was also occasionally revving high. For a while all was good but now the misfire seems to have returned along with the occasional high revs on start up and seems to occur when ever the car has been left unused for a while.
There are no warning lights and if i start the car again after switching off after misfire all seems to be good and it runs fine.
Just dont seem to understand why i am getting this occasional misfire?
Does anyone have any thoughts on this please as I would like to get this adressed before putting the car away for the winter.
The car has only covered 75000 miles with FSH and the last service being done at 64000miles around 15 months ago.