Thanks all for your input. Here is the update. I had to order the overflow valve and picked it up Friday. I got it put on this morning after which I took it for a drive and hammered on it. Through multiple full throttle runs and full throttle roll on's in 4th and 5th gear the worst I saw has just under 10 psi. On the other hand the best it would do is about 13 psi when running down the highway. Idling is about 15 psi. Although it didn't drop to 0 like it has been, it is obviously still not right.

My bet at this point is you guys who are pointing to a problem at the pick up in the tank are right.

John,
Yes it is still mounted on the side of the engine. The pickup has just under 230K on it now. Since I just replaced the stock pump and highly unlikely to shell out the coin for another, I am curious as to what you did for moving yours back closer to the tank. Do you have any pictures or can you point me in the direction of resources to do this?

The picture below was taken 3 or 4 years ago at Yellowstone and shows what the pickup now sees most of its miles pulling.