We have a somewhat large gaurdian farm and we use hopper minecarts to collect drops.

sometimes, minecarts will stop at random; regardless if we have an item above the hopper minecarts or not.

I hope this is just user error, however it does not seem like there is a way to keep the minecarts in motion.

There are 5 powered rails with 15 powered rails in between and they loop around.

I may not be providing the information that you may require please let me know and I would be happy to provide any info I missed.