i reckon this is the way its supposed to work. google drops sites in and out of the SERPs depending on what time of day/week it is. it makes sense if you think about it.
if you look for train times in the morning, then they probably want today's train times. but if they search for train times in the evening, then they probably want tomorrow's train times. so google will return different results depending on what time of day the search is made.