Usually when I see a leaking upper plenum, I prefer to change the intake itself out. I figure the gasket is about 75.00 itself, and the whole deal from carquest is about 127.00 or so and it comes with a new gasket. It also comes with the EGR tube modification to prevent them from melting threw and flooding and hydrolocking the engine up.
Just my .02 cents. It sounds like you have a external leak from what you are saying although you could possibly have both. It could be leaking externally and also leaking into the engine. But usually when its leaking internally there is a mis at idle and most of the time they just go without much of a warning. I would also carefully examine the area around the egr tube when you pull it off and look closely for any pitting around the 2 coolant ports. This will cause problems for you, if it is pitted, make sure vehicle is in in a heated area and use a cold weld or jb weld on it and fill in the pits and then later on after its cured sand it off with a really fine sand paper and then make sure to do the egr tube mod and reassemble. I know there are some cheap new upper plenums on ebay, but i never bought one so I don't know the quality or if i would trust it