Lets start by not locking down any port speeds, unless the effort is perfectly matched on the other switch, and even then I would avoid it if at all possible.
Was the link light not on before locking down the port speed?
What are the dip switches inside the DMC-700SCs set to?
you mention 3 Media converters to GBICS, are you running a LAG of some description over the link, or are they for different VLAN uplinks, or why the multiple uplinks?
What we need to figure out is if the problem is seen with the fibre run(s), the converter(s), or the switch(es), or some particular subset of your current full scale deployment. Test each of these individually as best you can.
If you just hook PCs to the converters can you ping?
