Causes of a framing nailer driver stuck in down position?

I have an Hitachi NR90AC5. It's oiled and has adequate pressure. There's something that's gone wrong with it recently, where it sometimes double fires (in single fire mode) and then the piston gets stuck down. I cannot push the piston back up by hand with the air disconnected).

I took it to a place, and they said it just needed cleaning and they cleaned it. But after I got it back it nailed off two more boards then stopped working again- same issue with the piston getting stuck down. I have an identical gun that works fine, so it's not the "cold" weather. (It's in the 40's-50's).

This gun was purchased in April. I'm just framing my own house, so it's not used that heavily.

I've repaired my finish nailer before, but I need to know what part to replace and where on the gun you open to fix it. It's not always obvious that a "bad" part is bad when looking at it. On my finish nailer, the bad part looked just fine. But after replacing the part, the gun worked correctly.

Does anyone know what part or parts to replace on this framing nailer that causes the symptom of a hard stuck down piston (sometimes preceded by double firing)?