What codec are you using to encode the data streams? Maybe this is the issue...If I was to use say Divx 6.0 with insane quality settings this would chew CPU resources, especially if I was running say 4 cameras...Actually under those circumstances combined with motion detection I reckon most PCs would shudder!
Try an uncompressed video codec first off...
Also check that your chipset/video drivers are installed and up to date - VIA drivers are notorious for being installed and windoze not using them, doze sticking with the M$ provided ones. In this case you have to do a manual search for the drivers to use them for IDE/USB etc. Of course I have zero knowledge of your proficiency with a PC so I am assuming basic user level...(oh - I see you mention a network sniffer so you must be power/uber level)
Check your drivers, PC temps (maybe your heat is ramping up and the C1E/Speedstep functions of the BIOS are throttling back your system to protect it - if your CPU/FSB drops in speed the utilisation will go up), voltages (if your CPU voltage drops quite a bit under load, you probably have C1E turned on in the BIOS - turn it off)
Without knowing how Eyeline is coded, I would also say that there is a chance the motion detection routine runs on the encoded video streams, which would take a fair bit of CPU resources due to having to render the video streams after them being encoded. There are a few DVR programs out there that do exactly this...
Then again, everything NCH does seems to be streamlined - as evidenced by the small binaries they provide for their programs! So I could easily be wrong about at what point they analyse the motion...
I'm a fan of their ideals - simple programs that do what you want and nothing else, with no bloat.
So I guess I'm saying rule out any problems that might not directly be the fault of Eyeline first - BIOS, heat, drivers (MB/Video), video codecs. Rule all these things out and you have isolated the problem pretty well directly to Eyeline.
Hope this helps!