|Side fans are not helping, they are just upsetting the proper air flow through the case and over the components. A rear fan high up in the back set as exhaust is all you really need and a low mounted front intake fan may slightly improve intake (many traditionalists even do not like this one either) without disrupting the flow through the case and over the components. So try unplugging the side fans and see the results.|
By no other customized cooling system (stock?), do you mean the stock Intel heat sink with the preapplied thermal compound and the push downs to lock it in place? If so then just make sure that all four of the lock downs are secure. If a different heat sink then you may need to remove it, clean off the surfaces and reapply the thermal compound (possibly too much or too little):
By 'no overclocking until now', if you mean that you have not had any problems with the temperature until you began overclocking, but now you do, then you should check/try the above but you also need to post the settings (multiplier setting and anything else you adjusted, especially voltages), the reading from CPU-Z, and reported temperatures (all) from Speedfan at idle, after browsing for 30 minutes, and after some gaming or other intensive use so we can have a better 'picture' of what is going on.
You have to be a little bit crazy to keep you from going insane.