Overclocking my PT880 got best results with a PCI video card instead of a AGP card. Why?

Question

Hi guys,
I posted a complete review of my PT880 OC experience at the "Overclocking section of the forum: thread post PT880 OC results" : https://forum-en.msi.com/index.php?threadid=48006&sid=&threadview=0&hilight=&hilightuser=0&page=2
Some days of test with my new PT880 board and I was able to run my P4 (B?) 2.4Ghz at 18 x 154 = 2772 Mhz. I got this speed using a cheap PCI card XFX GeForce 4 MX440, from my old P3 computer. I also got system to work with higher FSBs, but with some unstable sound board issues that I related on the link above.
This week I decided to change the card to another same cheap XFX GeForce 4 MX440, but now a AGP 8x version. My surprise is that the system started to hang with just 20~30 seconds of use. Setting the front side bus to 133 helped to fix the stability issue. Playing a bit more I could OC using a 144 FSB, getting a final speed of 2592. Trying higher FSBs results in a unstable system again.
This makes me wondering if the AGP/PCI is really locked. Using 144 FSB, it is supposed that an unlocked AGP port will be running at 72 Mhz, what is the limit for some AGP cards... My BIOS setup is using the AGP clock option at the default of 66Mhz. Is it really working at this speed?  
If the AGP/PCI bus speed is really locked in this VIA PT880 MSI motherboard, why this difference in OC results when I change from a PCI to an AGP card (same manufacturer, same nvidia chipset) ? Will be the northbridge more occupied now?  
My PC setup now:
- MSI PT880 LSR with Bios upgraded to 1.4
- Pentium 4(B? 533Mhz) 2.4Ghz
- 512Mb DDR PC3200 NonECC ADATA Memory (just one module, so just single channel)
- XFX GeForce4 MX440 AGP8x
- Seagate 120Gb 7200RPM ATA100
- Seagate 80Gb 7200RPM ATA100
- Prolink PlayTV HD (conexant cx88)
- Sound Blaster Live! 4.1
- LG DVD-RW 4040b
- 450W generic PSU
That's all for now  

Answer 1

Your AGP bus uses more bandwith than the PCI bus, thus making the OC more dependend from the AGP bus. And if your AGP slot will drain more CPU power and fragile as AGP can be, it will not tolerate errors due higher clock speeds. Mhz += Stability -. Notice: PCI is 33 Mhz/ AGP is 66/133 Mhz... . As you see there is less margin for errors. While you overclock the CPU will make tolerable errors: NP with PCI... But AGP... will take it to the MAX. [I hope this reply is No nonsense]