Hello,and welcome also to AF. As a moderator let me THANK YOU for starting a new thread
. Actually we are happy when someone starts a new thread rather than resurrect an older one. It's in the Community Guidelines as well.
I would take the van to a shop that specializes in radiator flushes and get the cooling system thoroughly flushed out including the overflow tank. Replace the thermostat and the radiator cap as well. It has been found in the past that one source of air infiltration into the cooling system was attributed to a faulty radiator cap. Stant is a good brand to get as a replacement. I would refill the van with Dex Cool because it was specifically designed for your engine. A few years back it was thought that you could switch to the old green coolant which was ethylene glycol based without any problems. But now it is recommended to stick with the Dex cool which is totally different. It is an OAT (Organic Acid Technology) based coolant and has a different protection package built into the product than the ethylene glycol. A major change in the thinking of Dex Cool is to now change it more frequently than the original GM recommendation of 5 year/150,000 miles. It should be treated like the old green coolant and changed every 2 to 3 years or 30 to 40,000 miles for optimal effect.
I fit comes down to a faulty intake gasket you have a couple of choices. You can attempt the job yourself, or take it to a garage to do it. One "pro" of doing it yourself is that you can save money, the cons are the amount of time it takes to do the job( very labor intensive) and the skill level needed, ( should have a good knowledge of how a vehicle is put together.)You also need to get a manual such as Haynes (first choice) or Chilton. You can also opt for a factory service manual which costs more ( fairly reasonable on eBay if you can find one,)and are more detailed.
If you opt to take it to a garage, I would not recommend a stealership as their prices are way too high. Find a good independent garage in your area,( ask family ,friends, neighbors for names,)and go there. You can expect to pay from $500 to $900 for the job through an independent garage.
You are right in saying that people assume that he Dex Cool can last a long time and not need service when just the opposite is true. Keeping an eye on it and servicing it more frequently than was once recommended should give you many trouble free miles. Good luck.
EDIT: I forgot to mention that if you decide to tackle the job your self there is a wealth of info on this site in the Chevrolet Venture, Pontiac Montana and this Silhouette forum. Just use the search function to find the appropriate threads. A good start is this thread found in the Venture forum:
http://www.automotiveforums.com/vbul...d.php?t=321467