Slipping gears is the problem that Sam Allison is having with his bike; this is quite a regular problem, especially in wet, rainy areas. His bike is 5 years old so there could be a couple of reasons for his slipping gears, we will try to explain
I have a Giant Mtn bike that is about 5 years old. Lately I have a lot of slippage while pedaling with any pressure. Sometimes it will act like it is shifting by itself. I am not sure if it is a gear problem or what.
There are a few reasons for slipping gears, the first is that there is either water or a piece of grit in the freewheel body, this can be cleared by running some oil into the back of the cassette body and hope that it runs it out the front, this doesn’t work every time and the body needs to be replaced.
As your bike is now 5 years old it could be that the chain and/or cassette are worn and need replacing, measure the chain to see if it has stretched and check the teeth on the cassette to see if they are worn and hooked. The chain tends to wear quicker than the cassette and should be changed more often; this allows the cassette to last longer.
The gears could be out of adjustment, first put the chain on the smallest cog and change from this gear to the next, if it changes precisely then there is no problem, if it will not go up smoothly and exactly then the cable needs tightening, if it goes too far then the cable needs loosening, this is done with the adjuster on the rear mech at the cable, turning one way or the other will allow the gears to change and sit in that gear properly.
My suspicions would be that the cassette freewheel is either worn or clogged in some way because of the weather, replacing a cassette body is not too easy and should be done by your bike shop mechanic. After 5 years I would think a worn chain or cassette are very likely reasons, if they need replacing you can do this yourself, but you need the correct tools for the job, if you don’t then again you need to go to the bike shop and have it done by them. The tools you would need are a cassette remover and a chain tool for removing the pin and shortening the chain.
There are your choices, so do a bit of investigating and you should have it sorted.