Dell Optiplex GX240 temporary freezes

[Rammstein]

I recently bought a Dell Optiplex GX240 from a second-hand computer shop, with the intention to upgrade it as much as possible, because it has seemed a reliable base to build upon (stable, well designed and carefully manufactured). It came with a Pentium IV 1.6 GHz CPU, 1 x 256 MB SDRAM, HDD Maxtor 20 Gb (probably 5400 rpm), ATI Rage 128 Ultra 16Mb, and an LG CD-ROM unit.

I made the following changes:

2 x 256 Mb PQI 133 SDRAM instead of the original memory
took out the HDD and the CD-ROM and replaced them with a Seagate 80 Gb (7200) Barracuda and a DVD-ROM Plextor PX-116A3
replaced the original video card with a Gigabyte ATI Radeon 9600 128 Mb

The HDD and the DVD-ROM are brand new, while the memory and the video card have been tested by me on another computer and perfrmed flawlessly for several months.

Now, my problem is: the system freezes quite often for 10-20 seconds completely (mouse not moving, keyboard not responding), and afterwards everything comes back to normal (no need to restart or anything else). Quite annoying.

I upgraded its Bios from the Dell website (it had version A02, now it's A05). I tried to lower the video card specifications (setting the AGP speed to 4x, then 2x, then 1x). I tried different AGP aperture sizes in Bios (256, 128, 64 Mb). I even went so far as to try and see how it would react if I put back its original video card - it still froze, so I placed back the Radeon 9600 one.
It runs on Windows XP with service pack 2.

What else should I try? Is it hopeless or does there exist some magic trick capable of making it work without these annoying temporary freezes?

Thank you

[saphalline]

How often does this happen? Every minute? Every 10 minutes? Also, are the freezes random or do they coincide with a certain action? Off-hand, I'd say there's either an IRQ conflict, DMA not turned on, or an onboard device that's not being used but still being polled by the CPU. Any yellow exclamation marks in device manager?

[Rammstein]

Thanx for your reply. From what I've been noticing, the freezes are random, but occur more often when I'm in 3d studio max or other demanding applications. As for their frequency, I don't know, let's say every 20 or 30 minutes. There aren't any yellow exclamation marks in device manager. What should I look at to see if there's any IRQ conflict please? I gotta see if DMA is turned on.

[Rammstein]

Indeed, DMA was off in Bios, I turned it on, but the machine still freezes just like before, at random moments, regardless of what is going on. Any other suggestions?