Well I just got a Presonus Firebox and I’m having some problems with it.

First of all, there’s a hell of a lot of background noise when I try to record with my SM57 (through an acoustic or an amp) which I can’t seem to get rid of.

Second, the recording cuts in and out. In other words it doesn’t record smoothly, it’ll do like 30 seconds and then it’ll clip and even begin to lag through the monitor. What do I need to do to sort that? I have a very powerful PC.

The first thing I would do is try to reproduce the issue while monitoring your system resources (cpu and ram usage).

Second I would make sure I have the latest drivers for the Firebox and for my Firewire card (or controller if it's integrated).

Maybe, check for updates for Cubase.

What OS are you running?

What kind of background noise? (Is it a hiss, a whine, etc..)
Irrelevant quote from obscure person
Obnoxious statement regarding size of e-ween
Italicized text indicating bandwagoning masquerading as deep thought
ASCII graphic that will take over the world if you put one in your signature
Made up statistic
Sounds like you have latency issues.