Hi! I'd remove the battery, Airport, and extended memory, and then reset the PMU and PRAM, and see if the behavior is any different before putting parts back in.
Does the laptop's video ever become distorted or go blank? Random freezing is a symptom of the G3 video chip issue, which is a logic board flaw with no reliable fix, and if the above doesn't help, I suspect you probably have a logic board issue.
There's a slight chance you could have a bad hard drive. To test for that, use target mode to boot from an external hard drive or another Mac, and see if the problem persists. Do all testing with unnecessary parts removed from the computer so they do not complicate the test results.