Greg,
If the back of the MPS-1 is accessible, consider spacing your amp modules in the following configuration:
blank, amp module, blank, amp module, balnk, amp module, blank.
This configuration should make the chassis more rigid than provide better circulation than having 3 modules next to each other and 4 blank slots.
If that doesn't work consistantly, then negociate a trade in for the 5 channel x 200 Watt amp they will have out soon or a discount on a couple more amp modules.