You can change the FASTSIZE value to 0 at the ISL prompt. This will reduce the memory tested to 256 KB. Just remember to set it back to 'F' when you are done.

Stan Sieler warned:

Although this may work, it's somewhat risky. Be very very sure to reset it to F as soon as possible. I wouldn't want to risk a boot-up after a power off if FASTSIZE isn't F.

Another option (if the performance impact isn't too much) is to pull some memory boards from your machine for awhile. I've done that on some models of HP 3000 when I was doing some R&D that required frequent crashes/reboots. The secondary advantage is that memory dumps are smaller (if you're taking memory dumps - otherwise it doesn't matter). On one machine, I used the PDC commands to disable five of the six CPUs, to dramatically speed up reboot times.