do you know what the fibre is that you are using in the backbone? ie multimode, singlemode and how old it is? some old fibres had a service life and would possible not support Gbit, as they are aging this can cause elements to drop out, if you are also expericening issues on brown outs this could also means that the actual light levels are low and then the loss across the cable could be too great for the switches to interpret. It maybe worth getting the link tested for performance as a link using a tester such as a Fluke or ideal tester which can check against an agreed standard, I would suggest either TIA 568c or ISO 11801 which would confirm if the fibre backbone could support Gbit.
May prove more cost effective that changing the switch and less distruptive as if required the new cables can be run, terminated, tested, then out of hours crossed patched over.
