Electrical outlet Tests good but not good
The three-light testers typically only draw a tiny current and usually use neon indicators that work at less than full voltage. They are good for showing a few common miswiring problems (e.g. where live and neutral have been swapped or where ground is not connected) but they are not good for showing bad connections which have a high resistance or which separate when hot.

I would turn off the breaker, verify power is off at the outlet and open it up and check the connections. The next step would be to swap the outlet with a new one or with another nearby working outlet.