The G-38 issues sounds like fuel delivery. Go back through the plumbing. It's possible to have a weak sensor fade with heat accumulation but there is no definition establishing magneto or electronic ignition. Timing is fixed with either one. If it's right at the start of a flight it will be right at the end of a flight, unless external scrws are visibly loose. So fuel delivery is the likely cause. Although the carb has been rebuilt it could have had something overlooked so include it in the fuel system checks.