PPC405GPr reset troubles

This may be slightly off-topic, but the hardware is as right as I can get it, so I'm looking for some sort of software problem.

I am having trouble with the "reset" command, and the /SysReset in particular, in connection with the EBC clock settings. The PSR (Pin Strapping Register) is spontaneously changing values on the /SysReset. In particular, the external bus clock divisor is changing from /4 to /2. I've added a dump of the PSR right where U-Boot prints the CPU speeds, and I get the following:

U-Boot 1.0.2 (Mar 5 2004 - 17:03:00)

XXXX PSR=0xfdfc0ce0
CPU: IBM PowerPC 405GPr Rev. B at 266.500 MHz (PLB=133, OPB=33, EBC=33 MHz)
PCI sync clock at 33 MHz, internal PCI arbiter enabled 16 kB I-Cache 16 kB D-Cache
[...etc...]
?> reset

U-Boot 1.0.2 (Mar 5 2004 - 17:03:00)

XXXX PSR=0xfdf80ce0
CPU: IBM PowerPC 405GPr Rev. B at 266.500 MHz (PLB=133, OPB=33, EBC=66 MHz)
PCI sync clock at 33 MHz, internal PCI arbiter enabled 16 kB I-Cache 16 kB D-Cache

I see with an O'Scope that the EMCTxEN signal (which is the difference between PSR values) is really low during the /SysReset, and it is clearly stable low then. But the PSR is read-only? Why is my PSR wrong?

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