If you really need them fine - if not comment out the src lines from your sources list.
The deb
http://apt.mepis.org/3.3/ unstable main was unavailable recently and changing to deb
http://apt.mepis.org/3.3.1/ unstable main resolved the problem. You can try 3.3.2. as well.
Most importantly doing a general upgrade is never adviseable and has caused many problems. When you say you have 135 that need upgrades, do you mean could be upgraded or 'need'? Do you ever in your life use all of them ?:-)
Best way is to use Synaptic - upgrade the ones you need to and even then look oot for any warnings and don't say yes when you mean no LOL.