What type of ground speed do you run on the 840?We have a MC840 that has a history pretty similar to yours. Hit something and it will break the gears in the cutterbar, if you're lucky the upper gear only, but it has broken several lowers and the whole cutterbar needs to be disassembled to repair that. I've gotten pretty quick with it, on ours I can drop one side of the cutterbar and have it slid apart in an hour or so.
We've only had this one for about 4 years now, but have been well pleased so far.
I'd like to try a NH next go round, Dad has an old 1412 and it pulls a lot easier than my yellow machines.