Reason doesn't use up that much RAM mrbishi... unless you're loading up gigs worth of Refills or you have 128MBs of memory or something. A CPU sitting at 100% is probably caused by a program going into an cyclic loop. Try checking to see if you've got something in a feedback loop (Virtual MIDI cables can do this.) Disable all VST plugins, MIDI utils, disconnect external hardware and then start putting everything back together. When you hit your problem you know what caused it and you can then attempt to remedy the situation. If it's a program, UNINSTALL FIRST, then reinstall it. Hope that sorted you out.

By the way, external sound cards usually cause a lot less trouble than internal ones since they don't deal with IRQs (Interrupt requests) or have PCI dramas. That is assuming your USB port drivers are functioning correctly :P