1998 Pontiac Trans Sport cooling system problems

My 1998 pontiac trans sport minivan has developed a problem where the dexcool coolant is leaking into the engine...my mechanic found coolant in the engine oil. I called my dealer, the man from whom I bought the car, and he experienced the same problem on his GM minivan. I did an internet search and found numerous others ([link removed] and other places) who have experienced this problem. It seems clear that the manifold gasket in these vehicles develops leaks when exposed to the dexcool coolant.

On 12/26/05 I smelled coolant when exiting my vehicle. After investigating, I found that coolant was leaking. I took it to a mechanic, and had the internal gasket replaced for $575. The next day I saw that it was still leaking coolant. I took it back, and had the water pump replaced for $166. The next day I saw it was still leaking, and I had no heat.