Help: Cracking/popping sound when monitoring or recording

[Spoony]

Hey guys, I'm having a real frustrating problem. Getting a crackling / popping sound whenever I record or monitor guitar or bass. Now, I know this is a problem when your buffer size is too low, and it's something that generally happens to me every once in a while. Normally I can fix it by just re-initializing Core Audio, but this time, nothing I have done has worked. Here are my specs:

2007 Macbook Pro
Intel Core 2 Duo Processor, 2.2ghz, Dual Core
2GB RAM
Presonus Firebox (Firewire interface)
Logic Pro 8

Sample

Now here's what I've tried so far:
-Checked levels on the interface input, as well as software mixer
-Closed all other programs
-Re-initializing Core Audio
-Changing buffer size in Logic
-Changing sample rate in Audio/Midi settings
-Restarting Logic
-Restarting computer
-Installing updates and restarting computer
-Changed guitar cables
-Changed guitars
-Tried new tracks, problem occurs with dry signal, no overloaded effects
-Bypassed my interface completely by plugging directly into computer's Line In, still there

I can play sounds from other programs, and even play software instruments and synths with MIDI, no problem, but when I record any outside instrument, there's that annoying crackling noise. Any ideas guys? Thanks in advance.