If it’s failing intermittently it almost certainly indicates a loose connection somewhere.
Are you using a probe or a clip on your test lead? If you’re using a probe, sometimes the slightest of movement can affect the result and give a high reading. A clip on the metal body of the appliance will prevent that.
Does it sit on a kettle base or is it supplied via a kettle lead? A bad/loose connection here could be the problem, or contact resistance could be the culprit on the base where the electrical contacts of the kettle & base meet. A high-current earth bond test may be more reliable here if your tester has that option.
Failing all that, there could be an issue where the earth connection meets the chassis internally
