Cubase SX 3 and CPU spikes

Hello, I am a new user!

First of all, my hardware and software configuration:
Sony Vaio with 1Gb ram, Celeron @ 1.50 Ghz, Windows XP SP3 (optimized: I got rid of superfluous services, “utilities”, unused drivers, startup elements and so on: the OS is very clean and quick in various operations)
Two sound cards (I am able to use both cards, inside Cubase, thanks to ASIO4ALL): a Roland UA 100 - USB - and the embedded audio card, a Realtek HD.
Well, also consider, despite the fact that the laptop is obsolete, that is quite able to handle complex audio projects (eg I have a Cubase project with 10 tracks with a lot of plugins), VSTi instruments, and the laptop doesn’t show troubles.

Ok: since I wish to play live music, I made a project with just two audio tracks (track 1: roland input with an equalizer - for the drum machine, and track 2 realtek input with an equalizer - for my synth keyboard, and a compressor on the audio output). Keep in mind that with this project, I never press “play” inside Cubase: I just run this project for mixing/live music. Everything works fine: I have a low latency (about 5 ms) and no troubles about synchronization; in the CPU monitor of Cubase I can also see that the consumption is very low: is near the first mark.
But there is a very annoying problem: after about 20 - 30 minutes Cubase starts to present CPU spikes (about every 2 seconds the CPU consumption goes from the minimum value to the “overload”: just this spike: no scratchings or glitches in the audio stream). I’ve found that, if inside Cubase, I select the option “Release ASIO Driver in Background” and then I minimize and I reopen the Cubase’s window, the problem will go away. So I think that the problem, maybe is present in the Asio driver? If I only use the external audio card and I disable the embedded audio card, the problem will never appear. So the culprit is the realtek sound card? Can I check anything else? I was thinking to get rid of the realtek sound card and install a PCMCIA sound card, which is far better: an echo indigo IO; would be a good idea or I risk to fall again in the same problem?

Many thanks.

Well, I reply to myself also to give help/hints to someone else which encountered this issue: I deselect “use system timestamp” under MIDI options in Cubase and this trouble seems gone: can someone explain me this reason?

Many thanks.

Use native ASIO drivers.