My memory is a little rusty when it comes to the XPS R model, but I recall some folks had a problem with the A13 release.  At the time, the A12 version was the one recommended by other XPS R owners.  Although A12 is no longer available for download at Dell's site, it is still available at Bob Matthew's web site which can be found by click here.

 

In reviewing the BIOS history though there isn't anything documented in the A09 through A13 updates related to processors.  There was a version A10 that wasn't made available for download that may have made a change or two.  It's difficult to say. On Bob Matthew's web site he makes reference to the A03 version that works with the Katmai Pentium III processor which maybe what you have.

 

I would also suggest visiting some of the links available on Matthew's web site. The link to Robert Hancock's Dell Processor Upgrade web page would be the best.  Mr. Hancock kind of wrote the book on processor upgrades for the older Dell systems back when the XPS R and XPS T models were popular.

The socket 370 1.1GHz coppermine Celeron on a super slocket III has no problems on this motherboard with BIOS A12, so it isn't a timing loop speed inhibit....