The vacuum lines that you are using for this Curious, what are they going to? Do you have any idea of how your intake valves look? Do you know if the seafoam treats you are doing are actually accomplishing anything? This and the timing chain issues are a major concern of mine regarding the engines long term durability. If you method works well, I'd like to start doing it myself to keep this problem at bay instead of havign to pull the manifold and blast the valves with walnuts. I do a crap load of around town driving, so I'm starting to wonder if my valves are gettign accelearted build up from being in high vacuum all the time. AFter all, intake valve stem seals is the primary source of deposits on the valve stems and why diesel engines don't suffer from this issue. Doesn't hurt that most diesels also use an open crank case ventilation too...
I just know my average mpgs on an extended trip dropped 3mpg compared to the same trip last year, valve deposits could be the cause. If the week end is fair, I hope to open the throttle plate and work an inspection camera down into the intake manifold and get a view of my valve stems at 20k miles.