The only thing I can think of is either your Voltage in the Car is creeping up too high, (Voltage regulator ????).
Do the lights become very bright when you excelerate, and become dim at idle ???
If this is not the case, then it would have something to do with either shock and Vibration, or Temperature (Doubtful, as its now getting closer to winter).
The inly other thing I can suggests is some of those Blue H4/H1/H3 Light Bulbs are a little bodgy, in terms of the Filament connection. I had my first pair of Blue ones Blow on me within the first month (Not happy Jan !), and a Friend had the same thing after 1 week. (P.S. I am talking about the Autotechnica ones here).
I am running Narva Blue Bulbs at the moment, with that extra Xenon stuff, and hadnt had any problems with em, but by the same token, I am not really impressed with them either ?
Hope this helps