DirectIn Response boxes on Mac OS 10.5/10.6

[susa]

In 2008, we ordered DirectIN Precision Button-Boxes (High-Speed USB Response Boxes).
Some of the response boxes are used with Windows (on a PC), some of them with Mac OS X (either with 10.5 or with Snow Leopard).
We are very happy with the response boxes as long as they are operated under Windows (on a PC).
With Mac OS X, however, we are faced with severe problems:
Obviously, the boxes are not reliably detected as proper USB devices. Sometimes they work fine right from the moment they are plugged in, but sometimes, no response button signal can be detected at all. We never get an error message.

Despite ample testing, we could not figure out the exact conditions leading to either success or failure:
- The problem is not due to a broken USB port.
- It is not a matter of plugging in the device before or after booting the system.
- The problem appears with OS 10.5 and 10.6.
- It is not a problem of computer type: The problem is observed with brand-new iMacs bot also with mac minis, MacBookPros etc.
- It does not help to use an USB hub (with active power supply).
- It is not a matter of whether there are other USB devices plugged in or not.
- We also tried to boot an iMac (where the box was not working before) via firewire from a MacBookPro (where the box was working before). The response box was plugged into the iMac: As a result, the response box did not work - so, it seems to be rather a problem close to the hardware (USB ports) than to the operating system (?).

It is also strange that the information listed in the system profiler is not indicative of proper functioning: The response boxes might appear there (as "DirectIN Responsebox" device) or not, this does not predict whether they function or not.

As we need the boxes for our experimental work, we really appreciate any help. Thanks!