I have a 4620L 3G only service, I am in a fairly remote area, but have stable 50% RSSI @ -90dBM signal. I've been here 10 days, the first 8 were fine ... slow, but stable. In the last day or two, I have horribly slow speeds, and frequent IP layer disconnects. TCP sessions reset all the time today - I'm trying to work. Simple SSH/CLI work and email.
Anyway, what type of stability and performance should one expect with that signal level? Just generally speaking. I do have an external antenna, which improves the signal from ~38%.