Last night I was messing about a bit on the computer when I got the idea that the temperature of both the processor as well as the case itself was running quite hot. The Pentium 4 (2.53Ghz) was running at 68° Celsius and the case temperature was up to 36°. It had been fluctuating a bit over the past few days, but I decided to check it out anyway. All fans are working as expected, so I didn't really have a clue as to what was causing the higher temperatures. I took off the left hand panel and this morning the core temp was down to 64° Celsius and the case cooled down to 31°!
Was it possible that airflow was so bad? When I woke up this morning around 1PM I took my dremel out and started widening the air sleeves on one panel to improve the flow of fresh air into the case. Let me tell you that a dremel may not exactly be the right tool to cut through computer cases, but somehow I got quite a bit done before my cutter was reduced to nothing but a metal rod. A quick removal of leftover metal bits - never wise to have those around computer equipment - later I took the vacuumcleaner out and got rid of as much dust in the case as possible.
I just replaced the panel and the case temp is already back up to 34° Celsius, which is still 2 degrees better as previously. Core temp is at 65° right now, although I expect it to rise to 66°, possibly even 67° again. If the little mods I did don't solve things, I may have to take it a bit further. Adding another fan is always an option, but I prefer to avoid that to keep noise levels down. The damn thing is sounding like a jetplane as it is!