I am now 99% certain that the files are just overwhelming the G4 with their size/resolution. I believe the solution is likely to be that we publish an 'SD' version at a considerably smaller size either 960 x540 (probably still too big), 854 x 480 (reasonable) or 720 x 406 (starting to get iffy in terms of reading the small LR UI text). I am working on this and will let you know what my plans finally are.
In any event, it is going to take me a few days to complete all the re-encoding, so it is likely there will not be a proper solution until Monday, Aug 3rd at the earliest.
My apologies to those who have had problems - mea culpa. I let the good looks of the higher resolution get away from older machines...