The 07 420 had power steering? I wasn't aware they added PS to the rancher until 08 or 09.
In any case, the PS on ATV's is electric. If it over heats it will cut off, so look for anything that might cause it to get too hot.
ADDED: Yes, looks like 09 is the first rancher with PS....they put it on the 07 foreman first to introduce it.
If you have a rancher and it's only a year old, take it to the shop as it's probably still under warranty.