Give us some idea on how it sounds when it starts to stumble after warming up
it sounds fine they it sounds like its going to stall and it does or it will continue running normal but the tach goes to 0 which I would assume is because its idling too low to register.
When I hit the high idle switch it stumbles for a bit like its got air in the fuel but after a few minutes at high idle it seems to run fine.
I think I really need to make sure that orifice is in place and replace all of the fuel lines and filters and see what it does.
The other thing I forgot to mention is that the jake brake doesn't seem to work as good as it should. It seems like its on low all of the time. It has 24-28 volts like its supposed to.