What is the size of your case? If it is too small, upgrade to at least a mid-tower with a place for a 120mm exhaust fan in the back. Use the largest fan that will fit the case holes, moves a lot of air (higher cfm), and is not too loud (lower db).
If you have good case air flow, then you need to address your heatsink and fan on the CPU. Replace the heatsink with a highly rated model that can hold a large fan (90-120mm). Remove the old thermal paste and apply Arctic Silver 5 as per its instructions.
Firstly, I apologise in advance that this is going to be rather lengthy, but I feel that if I supply all the pertinent information I can think of, I have a greater likelihood of a successful resolution.
I have assembled a system based on a Pentium 4 630+ CPU and an Epox EP-5EGA+ motherboard. At idle, with an ambient temperature of 22C, I am experiencing a case temperature of 35C and a CPU temperature of 42C. However, under maximum load, the CPU temperature increases to over 60C, at which point thermal throttling is activated (Intel specifies a maximum operating temperature of 63.6C). The case temperature remains constant at 35C. The fan speeds also remain constant at around 2500RPM, despite the fact that I have enabled CPU fan auto control in the BIOS.
I have the standard arrangement of an intake fan in the bottom front of my case and an exhaust fan at the back (the mounting position for which is located about halfway down the panel). When this didn't have the desired effect, I added another intake fan to the only remaining mounting position, which is on the side panel towards the back of the case. This mounting position is actually located slightly higher than the rear exhaust mounting position, which I suspect is not ideal, but it is also almost directly opposite the CPU. This fan resulted in a marginal improvement, but it certainly did not solve the problem.
I'm running out of ideas as to how I can cool my system. I'm using the supplied fan/heatsink combination and the pre-applied thermal interface compound. While I would not be adverse to a more drastic cooling solution if it is absolutely necessary, I would expect the standard fan/heatsink to be adequate seeing as I am not attempting to overclock. There is nothing blocking the airflow from any fan, and the internal cables are bunched together with cable ties.
Two possibilities have occurred to me. One is that the cooling fans are not responding as they should be to the increased CPU temperature. The other is that the supplied thermal interface material is not forming a complete bond between the CPU and the heatsink. Can anyone lend weight to one of my suspicions, or offer an alternative explanation?
Thanks for bearing with me. All advice gratefully received.