Hello from NH. I"ve read all your posts and I can definitely relate to the frustration as well. I have a 2004 EC35 with enclosed cab, and been having the same issues as described in different posts here (runs fine and then stops, runs fine and then controls go dead (except the blade) but engine on, won't start at all and control panel looks like a christmas tree, etc.). I did think it was voltage as well because I would jumpstart and it would sometimes work. Replaced the battery with best available....worked a little, then started acting up. Jumpstarted and that did not help (wouldn't even crank, or it would do a 1/4 turn). Called Volvo dealer and requested field service.....It's never good when the guy shows up and he has to look thru the manual for 30 minutes before doing anything.... He eventually verified all earthing (6) and clean them up. Told me it should do it, or else it would be the Puma control..... It worked for a bit and then started acting up again. Now I;ll probably try and buy the Puma control box as well, which was quoted at just a bit over 3k....
A friend that has 3 excavators (small excavation company) says he had a imilar issue with a Cat,and it ended being just a faulty wire....That's a lot of wires to change to go by elmination process...I<ll check the web in France to see if parts could be cheaper...Will post back with findings