25 HP Intek Valve problem

Have a 25HP V twin Intek that had me stumped. Low hours and always well maintained since I got it with less than 100 hrs. Head fins were clean when I got it. Used it for several years on a wood splitter with oil changes every year. Went to start it this year and only hitting on one cylinder. Had spark so I pulled the valve cover. Push rod laying there and valve was stuck. Guide was in correct position as I know overheating will make them move. Pulled the head and valve out and was gummed on the stem. Cleaned it and polished with 2000 grit and cleaned the guide. Oiled it and moved freely. Reassembled and ran fine. Next time to start, same thing. Pulled the head and noticed some build up in the intake tube and thought maybe valve wasn't seating so I lapped the valve and cleaned/oiled everything again. Checked the valve for wear on the stem with a mic and not .0002 variance. Back together and ran fine. Before next start up, pulled the cover and intake valve stuck again. Got fed up and got another head and all is fine now. What could be the issue with the valve doing that? This one has me stumped and I hate to lose. LOL
 

The first time I freed it up I ran it 5 hrs straight. Next morning it was stuck. Pulled head, freed up lapped valve and polished stem ran it 4 hrs straight. Pulled the cover before even trying and stuck again. Since putting the used head on I've had no issues and gone through 15 gallons of gas. Just never ran into this before in all my years and was wondering if someone else had. Has run great since I got it and haven't has to mess with carb or fuel stuff.
 

I could see that if it was new but I bought the engine used and it had 140 hrs on it. I've been running it 8 years since. After cleaning and polishing the stem, wiping with some oil, the vlave will drop in with gravity. Valve seat on original head was never overheated as the guide had not moved on it.
 
There was some blow back evidence( small amount of carbon build up) in the intake port. That is why I thought maybe the valve was seating and I lapped.