Why is my freewheel slipping? My freewheel tarted slipping this week. It slips most from a stop with no pressure snapping to significant pressure on the pedals. Slipping may be the wrong term as it really isn't engaging. Once it "hooks up" and pressure is continuously applied there is no slippage.

Bike is a 3 years old, kona humu-humu, and used for daily commutes in Montana. Stored in an unheated shed at night, left out in the elements during work M-F 9-6, a couple times a month year round while out on the town.

Past couple months I've been hearing a banging sound out of the rear cog which I assumed was the chain slipping on the cog, but now I'm wondering if it the ratcheting mechanism in the freewheel. 

Recent weather included a large snow storm which caked the bike in snow/ice followed by a week of sub 0F weather, so the snow stayed caked on. Two days ago riding it home after dark in like roughly -20F temps the issue first occurred. Thinking it was a chain slippage and being very intermittent I brought it inside got the bike warmed up/dry tighted the chain and did some other routine maintenance. 

Next day, same issue occurs but easier to recreate while off the bike and I can see the chain is fine and the freewheel just isn't hooking up. 

I have a couple theories but not how likely each is. I'm also not sure how serviceable the freewheel is. 

Theories:
- Water worked it's way into the freewheel and is now frozen inside the freewheel. This is changing how the moving parts operate.
- The banging sound was the freewheel and is a sign of or the cause of major wear. The mechanism providing traction now has a loose fit and the extreme low temperatures are enough to further loosen the connection to unusable.
- It's too cold for the springs/oil in the mechanism to perform adequately for normal usage
- Something else?

I need to swing by the local shop anyways as I'm swapping out some parts shortly, but if there are some simple fix ideas I'd love to hear them.

EDIT: Freehub not freewheel + Single speed cog.
Since this was mentioned in a comment, freewheels are not the same as freehubs+cassettes (see http://sheldonbrown.com/free-k7.html) for details. The rest of the discussion should hold if freewheel is replaced with cassette+freehub though with minor modifications - I don't know if this bike has a freewheel or a cassette+freehub since I can't load the product page. Freewheels and chains (which needs to be measured with a ruler, due to some issues with most chain checking tools according to Sheldon Brown), (and less frequently, the chain rings) are wear items - they don't last forever (and the ratcheting mechanism can go and make a lot of noise). In 3 years, you may have exceeded it in commuting depending on weather and mileage - Last freewheel I had lasted about 3500-5000 miles (some cheap sunrace freewheel) since I ran a while with a worn chain. You don't fix a freewheel unless you have to - theres a lot of bearings and what not which makes the complexity high. Buy a replacement freewheel and have it installed, and throw the old one away (and usually, thats a good time to replace your chain as well, though you should get at least 2-3 chains / freewheel). 

I wouldn't call it a banging noise (I haven't thought of what a good description of what the noise would be), but hubs which need repacking or axle damage also do make noise, so you may want to check that too. You can also get a banging-clunking noise from a failing bottom bracket as well (which can lead to a slipping sensation), so checking the bottom bracket area is another thing to do (in doing this, you'll also have to check the cranks and pedals, so this is a bit of work).