Okay, first solution... chill.
Seriously. It gets frustrating, sure. One of the hardest skills I've had to learn is when to walk away, have a cuppa, and just chill...
Right, so, we're at hte point now where we know the signal going into the copmuter is fine. This rules out the mic, the cable, the preamp.
This leaves:
AD converter, Firewire card, drivers, software, hardware, DA converters.
What are you using as a playback device? If it's your internal soundcard then that's likely to be the culprit. (I suspect this to be the problem). You should be playing back through the firebox.
From what you're saying, it sounds like you're overloading your computer somehow. Are you using ASIO out of Cubase? If so, try increasing the buffer size. This will increase latency, but hopefully increase stability.
Out of interest, what bitrate (bit depth/sample rate) are you using?