It would be better if you post your PC spec...
Anyway, many factor can cause track artifact.
Motherboard:
Chipset issue, BIOS setting, Bus architecture, resource (IRQ, DMA)etc.
HD:
HD speed, disk controller (bandwith), cache size, etc.
Soundcard:
Driver, A/D and D/A, latency setting, etc.
Software:
Plugins, etc.
Here you can see, even though Sonar is the best on the market, you need powerfull machine with perfect combination to make it run smooth. My older MIDI DAW has VIA chipset which always start making noise after 10 or more audio (DXi) tracks.
Then what can I do, Jaymz?
First, make sure you install latest -best working tested- driver necessary. Motherboard, soundcard, VGA card, etc.
Defrag your HD at least once in a week if you're extensive PC user.
From Windows control panel, check if your soundcard shares resource (IRQ, DMA, etc) with other devices. Move the soundcard to another available PCI if it does. You must know strength of your machine. I mean, how many audio track it can handle, and how many plugins. Keep your eyes on the CPU and Disk usage meter. If only a few tracks (below 8) then DING! Time to upgrade...
Then what should I get, Jaymz?
Get better soundcard at least from Audiophile 2496 class.
Get better HD (7200RPM) if you didn't have one. Separate OS HD and audio HD physicaly (yes, that means two HD on two different IDE).
Get more power! At least 1,8GigHz Processor and 512RAM...
Lets see if it helps...
Jaymz