I agree whole heartedly with Sheerwater. Should have mentioned HT leads & distributor cap in my own earlier post.
You could also try spraying WD40 all over the HT leads & distributor too. That always works on my wife's Renault 5! But, I suppose that if it does work it really means you need a new set of HT leads at least & possibly a distributor cap. Try cleaning the cap first though like Sheerwater says.
A set of new HT leads is only about £20, Bosch ones are good.
Another thing that can be checked for no expense is the earthing points that affect the engine electrics/electronics. If they have corrosion that might be having an effect too though quite how or why damp weather would cause it to be worse, I don't know! There are wiring diagams at:
http://www.volvoclub.org.uk/wiring/U...%203905202.htm but you need the current user/password from the Driver magazine to access it. In the left-hand index column there is an item for Ground Connections and shows pictures of where they are, which makes them easier to find.
HTH
Andy