21" iMac Windows sometimes won't boot issue.

I am looking after an environment of about 40 21" iMacs setup to dual boot either OSX or XP.
This works perfectly almost everytime on every machine except 3 or 4.
The problem machines work mostly but every now and then (perhaps 25% of the time) they will stall while trying to boot XP, either on the gray initial screen or with an error message just after the white blinking cursor.
Normally a power cycle fixes this.
I have tried everything right back to clearing the PRAM, wiping the HDD and reinstalling and configuring both OS partitions, but to no avail.
OSX works perfectly on the affected machines its only XP that has the issues.
Have tried a number or ways of the selecting the OS from holding down ALT to Bootpicker or Refit, always with the same inconsistent results and always only a problem on the same few machines.

Any ideas...

I don't have a solution for you but on my machine, dual booting between Snow Leopard and Windows 7, I'll get a bad boot to Windows 7 every once in awhile.
It corrects itself on a reboot. I have no idea what causes it. Gremlins ?

Regards.

Yes its very odd and only affects approximately 10% of machines, the other 90% are very reliable in this regard.