Your problem sounds to me like the Auto Gain in Serato is confusing you. i´m right ? Try Disable Auto Gain in Serato Settings.
Make Sure your Latency is Set up Well and no other background tasks are working.
Are you Sound Location´s Completely Analyzed ? (Since SERATO 1.6.1 there is no way to RE READ ID TAG´s by your SELF - SERATO DOES it where you PLAY LIVE!)
I Run Serato on an 17" HP-ProBook - Intel Core i5 (m480) 8gb of ddr 1333 rams and an SSD 240gb from Intel and Windows 8.1pro
and on my Workstation at Home Intel Core i5 (760) 4GB ddr 1333 (a lot of hdd´s of course) and there I figured out that the Newer USB Drivers from INTEL Brings up new Energy Settings as Default, you have to prevent SERATO from any ENERGY SAVEINGs that RUNNING on your System!
For Windows Look in Energy Settings set up to High Performance - look in your Device Manager for Energysettings in the Driver ( your LAN Card has an WAKE ON LAN feature some Drivers combines that with Save POWER when DEV not needed. SO deactivate that and Magic Packet eg. Will not WORK BUT your LAN CARD wont TURN OFF.. look for the same in USB ROOT HUB.
Did you Tried DPCLATENCY ?