A jetpack , like any other cellular device, requires a good signal. Have you verified you are getting a good signal to the unit? An app like Opensignal will help determine what towers are near you and what sort of signal strength you are getting. Throttling wouldn't be intermittent. Once you reach a threshold it will slow down until the next cycle. I know on our last jetpack it was pretty finicky about where I placed it in the RV. I bought a small antenna for it and that really helped. Also, if you are happy with your phone as a hotspot, what do you need a jetpack for and why did you change the sim card?
Years ago, I had a hotspot that worked great, until it didn't. No idea what happened and it was replaced by the carrier, but it just stopped working properly. Very weird.