p6vap-a+ with slow UDMA

I've got a p6vap-a+ PIII 866 with a WDC UDMA 66 30G harddrive and W98 SE..
HDTach is claiming that my Read Burst Speed is on the order of 22 MB/s. I'm
expecting it to be in the neighbourhood of 66 MB/s. BIOS detects the drive
correctly, DMA box is checked for the disk drive in the Device Manager.

I've tried updating VIA drivers etc. without change. Western Digital
on-line diagnostics claim my drive is working fine.

Am I mistaken on the expected burst speed? Any suggestions for getting the
transfer rate up?

Thanks,

Paul.