RonRonMTB, I have the same bike as you and have the same motor cutting out issue. Like you I found that once it starts occurring, it becomes more frequent and in my case, it eventually fails to turn on at all. Also like you, I found that if I removed and reinstall the battery it it either does not recurr for the remainder of the ride or its delayed significantly. The problem is indicative of a bad battery connection but neither me nor the bike shop has found anything amiss.
This is what has happended in my case:
-Giant initially seemed to think it was the charger and gave me a new one (didn't make any differience)
-Giant then recommended that I deplete the battery to 4% and recharge, (I depleted the 3%) I assume that this was to reset the Batterie's Management System, BMS (but this didn't work)
-I swapped my battery with my friend's battery (we have the same bike) he did not experience the problem nor did I (although these rides were interupped with flat tyre fixing and some mechanical issues....and as the problem usually only develops after i have been riding hard for an hour or so ...I am not confident that these were good tests)
-Giant then provided me with a replacement battery to try (the problem didn't recurr)
-Giant then requested I reinstall the orginal battery, and so far the problem has not recurred on that battery either (but I have only given it one really good run where I would have expected the problem to occur)
I now have a motor issue (possible stripped internal gear) so I can't do any further testing until a get another motor (under warranty... I hope).
As with you I'm puzzled, especially now that the problem seems to be gone, Giant wants the replacement battery back but I have still not done enough rides to be confident that it's now OK.
I do have a possible theory as to why the original battery now seems to be ok. On the last ride before the replacement battery arrived, I ran the battery down to 0% (fully discharged), this might have done the BMS reset that Giant was seeking but required a fully discharge battery rather than a 4% discharge?
Just out of interest......I have measured the battery output terminals and it does not have any significant voltage on the terminals, indicating that the battery has some internal switching to only power the terminals when it is signaled to do so (I assume this is a safety feature). So for the fault, it is possible that could be caused by:
-interuption of the switching signal to the battery to keep it turned on,
-the BMS might think there is an internal problem and is switching the battery off,
-there is a fault with the switching components within the battery
-there is a bad connection between the battery and the bike
-there is a fault with the handlebar controller and it's incorrectlhy sending a signal to turn off the bike.
-or something else?
I woud be interested in your experiences and what your final solultion was.
By the way Gaint has been hepful and striving to sort this issue out .....Cheers,