My idles jumped from 29 to 36 over the course of the last few weeks or so. Given that hotter initial temps will lead to higher losses as the differences between air temperature and the heatsink change, it gets harder to transfer heat and equalize temps, its entirely plausible that your 'sudden change' is indeed the consequence of summer.
Get speedfan and keep the fans running at a constant speed in the higher percentiles, as high as your ears comfortably allow. Mine starts whining beyond 80% so that's where I keep it at to cope. Also, seeing as though you're not overclocking the CPU it might be a wise move to experiment with undervolting it slightly, it would help a great deal. Most Q6600s clock well even on stock volts so you can probably get away with giving it less juice, they're not known for running cool to start with tbh. Besides, manufacturers give CPUs more volts then they need as a rule to ensure rock-solid stability. Play around with it....
- Ordokai