All software available by Nokia for 6310 is available via http://www.nokia.com, currently there is no .mdm file seperately available. Please check out http://www.nokia.com/phones/productsupport/6310/index.html .
I had the same problem, but solved it by taking an MDM file for 6210 and editing it. I started with the 621096.MDM file and kept looking in the Notes log to find the lines which were causing the problem (look for a modem response of 4 = Error).
I simply commented out lines which were causing problems - references to S35=7, S34=0, S9=6 were the main problems.