I have a 2004 Sportsman 400 that acts like it’s possessed. I bought it new from a dealer. It has 800 miles on it, sat for two years with gas treatment in it. I took it out this year and have had stalling, missing, power loss, etc. So I did the following

removed tank, blew out screen with air
new fuel filter
new gas lines, removed and blew out all others.
new fuel pump
new air filter
new air lines to air box
new air box gasket
rebuilt the carb. using a hanes manual
new spark plug wire and spark plug
adjusted valves

I brought it to a Polaris dealer, and a former Polaris dealer and the missing, stalling, power loss continues. Does anybody have an idea what else could be wrong like electronic issues?