I'm not an IQ expert by any stretch but I doubt that it fluctuates that much (in the population in general). Based on the limited reading I have done your IQ will tend to rise as you get older, however in general people will stay in roughly the same position relative to others.
Whilst you can probably influence your IQ by working your brain really hard every day or lower it by doing a lot of drugs I doubt it is really that meaningless as it does tend to correlate well with success.
However I'd be curious to know how it correlates with lifetime income once you control for family income. I assume this has been researched.
Since we are making anecdotal observations I'll posit that you could significantly increase your IQ if you focused on it. Just like if you wanted to play an instrument. You can increase your playing ability by focusing on it, or more specifically, by practicing. That is how I see IQ, as something that you can get better at by practicing, nothing more, nothing less.