We used mobile Internet service (HSPA and similar) as our primary household Internet access for several years before wired DSL finally arrived in the neighborhood. Our hardware was the usual 3G USB modem stick plugged into a CTR-350 Mobile Router (a wifi router with a USB slot to accept the modem stick). This was back when these services were just becoming widely available.
It was perfectly reliable from a service stability point of view.
Modern smartphones should be able to provide the same function. My iPhone 3GS doesn't offer the WiFi hotspot option, but my laptops can still connect via Bluetooth with the phone in my pocket. Magic. Newer iPhone 4 can offer the wifi hotspot version.
More than a few years ago, I rigged up the CTR-350 in my car and streamed BBC World Service to my laptop. An FM modulator finished the circuit to the car radio. I confirmed it was dead reliable over a 25 km route, but not very far outside the city. I also confirmed the link stability at the specified max ground speed ;-).
(Gee - These days one just presses the BBC preset on the Sirius car radio. Some things are getting better...)
If you have a weak 3G signal, then perhaps you could use an external antenna. Problem is that "Ext Ant" have become very rare in the past few years.
If you're trying to use it on the move, then perhaps the connection is being handed off in a haphazard manner by the network. I'm not a networking expert, but I know enough to know that the network operators can make a right royal mess of things.