issues with multiple firewire devices

Over the past few months, I've noticed issues when daisy chaining FW devices to my MBP (a 15" purchased September 2009. Specs are posted in the signature). It often seems that a third device causes connectivity issues. 2 will always work fine (any 2) and of course 1 device is a no-brainer. But as soon as I add a 3rd to the chain, stuff happens, Specifics:

• This usually involves 2 hard drives and an audio interface.
• The audio interface has a FW 400 input. The hard drives have 800 and 400, among other options.
• Since I can always successfully run any 2 devices without issues, I have to eliminate the drives, cables, or the new audio interface as a problem (??)
• I'm not aware of any storm or electrical surge to have done damage. Anything is possible, of course.
• For my day-2-day work, I need both drives and the audio interface up and running.
• Data always seems fine.
• ??

The main question is are there any known issues with the FW port/chip on these MBP's. But also, how can I test the FW port integrity?

Thanks in advance for any advice.