A glitch

Ok so there is a glitch in the whole potty training thing- everything is fine if he is naked- no accidents or anything- he will go to the potty for pee and poop- no fear, no nothing. However, if he has undies or pants or anything at all on - then he doesn't go- he has accidents. Not all the time but during the late afternoon and early evening- as if his brain is too tired to remember that he is not wearing a diaper- can you say wearing thin on mommy? So here is my delima- I will be driving home with the kids alone to Nashville the week of Christmas- do I hold off and wait knowing that the excitement of being at my parents with cousins and friends etc... will set us back and don't even get me started on the 16 hours of driving there and then 16 hours back OR do I wait and concentrate after the new year? Any thoughts out there?