Laurence,
I have this happen frequently at some of our locations that are served via T1, mostly when they're maxing out their bandwidth. There's settings you can change for both the timeout and the retries available, just go into the properties for the device and click the SNMP tab and you can edit them there.
There's also a couple lines I see in the defaults.inc.php for this, however they are commented out so I'm not sure if they're deprecated or possibly just so it defaults to the individual device settings, as I don't use them. If you want to, add these to your config.php file with whatever values you want:
$config['snmp']['timeout'] = 1; // timeout in seconds
$config['snmp']['retries'] = 5; // how many times to retry the query