Huawei mobile broadband E173 data card not working properly
I had a similar device. I will just post some insights I got using that.

Mostly I remember it didn't work when it wasn't plugged in at boot time. But I remember also similar problems as you describe.

Could you check next time it occurs to you quickly reboot and see if the problem persits?

Actually after some trying I found out that I could not only connect through the Huawei software, but also through the built in modem program of Mac OS. In the Network settings the modem showed up and I could connect from there - even automatically. This turned out less prone to problems. (How do you connect?)

Also I remember it was not easy finding current drivers (my provider didn't have any for Mac). In the end I ended up downloading the most current from another network provider and manually inputing the correct settings.