Either of your suggestions are viable and easy to correct.

Change the Plug and retune for a lower RPM, then go flying. Cost one Plug and ten minutes.

However; both problems should have been evident in the FIRST flight, I would also start looking at things like crud in the fuel tank that gets stirred up or loose fittings. An Air leak around the Carburator can cause no end of problems.

Basic fault finding, change one thing at a time and try to duplicate the fault. When the problem disappears recall what the last change was and do it first the next time.