HP 840C USB problems

I have n HP 840C, which works fine using the parallel port. I have a black laser on that, so I moved the 840C to the USB port. It prints text ok, it also prints low-res colour ok, but try to print a page at photo-quality and it prints 70% of the page fine, then stops with an ‘unable to communicate with the printer’ message. I am using the latest drivers from the hp web site, (PC has 128m Ram, 400mhz chip, 30 Gb h/drive)
Can anyone help please?
thanks – Paul