2014 Tundra what are the symptons of a broken valve spring?

Truck has a really strange intermitting problem. Sometimes when I give it gas to pass someone the RPM's hang between 3000 and 4000 rpm's and there is no increase in speed. So I let off, then I hear a thud. If I try to just hold it the check engine light starts flashing and the truck starts shuddering. I have also had it happen a few times with a head wind and going up hill on the freeway. I have had the codes pulled it always shows random misfires and bank 1 and 2 lean. I took it to the dealer and they have no clue. They are guessing that it's a broken valve spring but are not sure. What would the symptoms of a broken valve spring be? The reason I ask is because the dealer could not explain to me why they think it is a broken valve spring. The tech took a snap shot with tech stream and sent it to Toyota engineer's.