My case fan usually regulates properly by speeding up when the machine is under load and slowing down when it's idle.

Since I installed 8.04 it doesn't always regulate properly (two previous Ubuntu versions didn't have this problem). If the machine is put under load causing the case fan speed to increase, often it will stay at medium speed even after the CPU settles and cools.
If the machine is put under load again while the fan is stuck at medium it will speed up but then returns to the medium speed after the load is removed.
The only way I've found to correct the problem is to reboot. The fan speed settles back down when the POST screen is displayed.

I've tried lm-sensors (which includes pwmconfig and fancontrol) but it only detects hard drive, cpu and main board temperatures - no fans. pwmconfig detects no fans. CPU and mainboard temperatures return to normal 40-45ish while the fan continues at medium speed. I had to run /usr/share/doc/lm-sensors/examples/hotplug/unhide_ICH_SMBus to make lm-sensors detect anything.

The BIOS has no settings relating to the fans, temperatures, etc. other than the readouts.

I've found nothing under the /dev or /proc/acpi directories to control the fans, but there's a lot of entries in there and I may have missed one.

A proper fix would be great, but I'd be happy with a setting in one of those directories where I or a cron entry could manually reset the fan speed. So far I've not been able to find anything of the sort.

Please help!