It is definitely something to try. But the deviation is not the same (as noted above, sometimes it's -2000ppm, sometimes -1000ppm, other times +1000ppm). And I have neither scope nor counter. I'd like to get one of the TBS1022's though. $)
Actually I already bought some of the RTCs. I just can't work on them as I need to travel for the next week. But realistically the effort of putting in an RTC is less than digging around the code. And the added cost to the solution isn't that much, considering that it relies on an encoder that costs anywhere from $350 to $700. So a $5 RTC and crystal are insignificant.
Now if I still see drift after putting in the RTC.. then it would have to be a software bug. 