Sounds like maybe your buffer settings are too low.....that would also explain the headphone "skip & pop". But, It could be any number of things. Have you optimized your pc for audio production?
Have you disabled wi-fi? Did you install the drivers from the included disc? Or did you download and use the latest drivers from this site? Also not all firewire chipsets are created equal. Are you using a laptop with onboard firewire? If so is it a Presonus approved chipset? Not all firewire chipsets are compatible with ANY firewire audio interfaces. Check it against this list:
http://support.presonus.com/hc/en-us/article_attachments/203654653/Approved_FireWire_Cards.pdfThat being said we can help you better now and in the future with a bit more info......if you will post your system specs in your signature, (see my sig), we will be better able to troubleshoot your problems. Without knowing what type of pc you're running, what OS you're using, what DAW etc....we often can't recommend a "simple fix"
If you have not already done so please READ THE MANUAL!