Decreasing fps

[Ronsard]

This baffles me. I don't understand why my computer has such low fps with such specs. Sure, it is not a high-end machine, but it should be more than appropriate for the games I play. What I've noticed is that most of the time, when I've just booted the computer, the graphics are smooth. However, a few minutes afterwards, the graphics start stuttering. Restarting usually solves the problem, then graphics stutter again, etc. I've noticed this in Warcraft III, Civ4, FIFA 05, FIFA 06, Diablo II, just to mention a few. I've upgraded graphics cards, upgraded RAM, tried different hard drives, set up a 2Gig pagefile, set up no pagefile, reloaded OS several times, closing background apps, to no avail. Any ideas would be welcome. By the way, mobo has an integrated graphics card which it disables automatically when it detects an AGP card (don't know if this could be the problem).

Thanks!

Computer:
Operating System Microsoft Windows XP Professional
OS Service Pack Service Pack 2
DirectX 4.09.00.0904 (DirectX 9.0c)


Motherboard:
CPU Type Intel Celeron D 315, 2266 MHz (17 x 133)
Motherboard Name Foxconn 661M03-G (3 PCI, 1 AGP, 2 DDR DIMM, Audio, Video, LAN, IEEE-1394)
Motherboard Chipset SiS 661FX
System Memory 768 MB (DDR SDRAM)
BIOS Type Award (01/22/05)

Display:
Video Adapter NVIDIA GeForce 6200 (256 MB)
3D Accelerator nVIDIA GeForce 6200 AGP
Monitor NEC MultiSync A700+ [17" CRT] (9903387YA)

Multimedia:
Audio Adapter SiS 7012 Audio Device

Storage:
IDE Controller SiS 5513 IDE UDMA Controller
SCSI/RAID Controller SCSI/RAID Host Controller
Floppy Drive Floppy disk drive
Disk Drive WDC WD800BB-00DKA0 (74 GB, IDE)
Disk Drive ST3160023A (160 GB, 7200 RPM, Ultra-ATA/100)
Optical Drive OQ5301P KSY524P SCSI CdRom Device
Optical Drive SONY CD-ROM CDU5215 (52x CD-ROM)
SMART Hard Disks Status OK

Partitions:
C: (NTFS) 152625 MB (135338 MB free)
D: (NTFS) 76308 MB (14533 MB free)
Total Size 223.6 GB (146.4 GB free)

Input:
Keyboard Standard 101/102-Key or Microsoft Natural PS/2 Keyboard
Mouse Microsoft USB IntelliMouse with IntelliEye

Network:
Network Adapter SiS 900-Based PCI Fast Ethernet Adapter (192.168.0.101)

Peripherals:
Printer hp psc 1310 series
USB1 Controller SiS 7001 PCI-USB Open Host Controller
USB1 Controller SiS 7001 PCI-USB Open Host Controller
USB2 Controller SiS 7002 USB 2.0 Enhanced Host Controller
USB Device hp psc 1310 series (DOT4USB)
USB Device hp psc 1310 series
USB Device Microsoft USB IntelliMouse with IntelliEye
USB Device USB Composite Device
USB Device USB Printing Support