It may not be the same problem, but I've recently seen XF86_Mach64 lock up using all the CPU when I do a rubber-banding operation in a Motif application running remotely. I'm using a self-built but vanilla 2.2.14 kernel. For what it's worth, it doesn't happen on an RH 5.2 x86 box with an S3 driver. I haven't time to investigate at the moment.