100% CPU usage for no reason
No, it won't be for no reason. Common reasons, malware aside, are (a) anti-virus software (personally I don't run it for precisely this sort of reason) and (b) unecessarily flashy stuff in web pages.
Find out.
build-up of PF usage to beyond the 1Gb mark
What exacly do you mean by "PF usage"? Are you running some application continuously that has a memory leak?
when I’ve left the computer on for a couple of days
Oh. I don't do that, personally, I tend to have to go to sleep every now and then. Ever heard of global warming? - if you're not using it turn the ******* thing off!
(Unless it's a server providing a 24/7 service to the outside world, of course.)