Gas water heater runs hot and not

We have an 8-year-old gas water heater under our house in North Carolina. Two things go on with it that are strange: 1. The water is always hotter from the faucets in winter than in summer (reverse of what we might expect!) and 2. Sometimes for no apparent reason it will run cool for a few hours or a day (max temp from the hottest faucet just over 100F) then it will revert to heating the water normally (up to about 120, except in winter when it gets up to 130+). This happens with no intervention from us. What do you think is going on here, and what can we do about it?
The WH is insulated with a "blanket" if that matters.
Thanks,
Kirsten

Sounds like buildup on the thermostat inside of the tank; replace the thermostat and this should correct the problem.