Symptom:
Right after starting the squid proxy, the CPU load goes up and up and up and never seems to stop (we've experienced 20.00+ on a 300 MHz CPU) - which is abnormal, because the correct behaviour would take 0.00 CPU usage with normal load.
Problem:
Something is wrong with Squid's access to its cache (/tmp/squid-cache/). We've encountered this problem after changing the squid's UID.
Solution:
Make sure that the user that squid is running as has read/write access to "/tmp/squid-cache" (or wherever your squid writes its cache)