Activity Monitor Doesn't Show Memory Usage

If this happens when you are using Safari, then it is the culprit. It will cause your cpu to spike and stay at very high percentages after running for a while. This doesn't show up as a huge memory drain percentage. Have a look at your average cpu usage, that should tell you what's happening. The fix is to empty the Safari cache, but that's unfortunately, only temporary.

I've had problems with 10.3 and especially 10.4 playing very fast and loose with disk space, easily eating up almost a GB of HD space in temporary files presumably for virtual memory. The behaviour of the computer (in this case either a powerbook G4 or an ibook G3) becomes erratic - is it possible that you have very little space left on your hard drive (say, less than 700 MB just after a restart?) and that by the time you start experiencing your problems you are done to less than 100 MB? Maybe the fan use is indicative of a whole lot of disk writing going on as the system tries to deal with less and less disk space for its greedy virtual memory needs.

If you have less than 1 GB of free space, try freeing up at least that much space, and then restart.