I was hoping more experienced hands would have a few ideas for you. I didn't really have a clue until you updated the thread with the mechanic's findings.
As your engine warms up, there's differential expansion of the different types of materials. Aluminum expands more than steel. Parts expand and possibly twist or flex. This will especially be true if you have a blocked water passage somewhere, causing your engine block to cool unevenly. (I've had that happen.) Anyway, long story short, you could be developing a vacuum leak that occurs only when the engine is warm.
What the mechanic was doing was spraying a fuel in small quantities to "sniff" for vacuum leaks. Wherever there's a vacuum leak, extra air is entering, so the fuel/air ratio is too lean. If a fuel is introduced through the leak, then the mixture is enriched, and a difference can be heard in the way the engine is running. For instance, if there's a vacuum leak early on that leans the mixture for all cylinders, introduction of a fuel through the leak will cause the RPMs to increase. In your case, there seems to be a vacuum leak feeding specific cylinders, so introduction of a fuel causes the engine to even out.
I'd speculate your problem isn't an injector leak, but rather a leak between the intake manifold and cylinder head -- or possibly a crack or defect in the intake manifold itself. You might want to keep "sniffing" for leaks the way the mechanic was doing; however, you might try using an unlit propane torch instead. Whenever you hit a leaky spot with the propane, the engine should run differently.
Good luck with it!