Once in a while some MiFi's data usage is about to expire and this will cause the speed to be throttled once it hits the 15GB limit.
What we do is move the MiFi from account 0004 to 0001 or viceversa.
Log in to Verizon Account >> Select Phone >> Click on the Upper Right Corner on the I WANT TO menu >> Move Wirless Number >> Change account and Submit
Thanks,
Audel