I just bought a 99 lesabre and had the same problem within 3 days. I have the car back at the dealer. I have done some research since and found that the problem is in the design of the plastic Plenum. They are not necessarily cracking, but there is an egr valve that slips into a hole in the plenum. The egr easily exceeds 500 degrees and eventually causes the hold to deteriorate. Close to this hole are water jackets that cool the top end of the engine. Pitting starts to occur and eventually lets coolant be drawn into the lower intake. This is because GM wanted to save .20 by not putting a metal sleve in the plenum. Ken-co has designed a fix for this, but will not sell to the public. You can call them to find a dealer near you that will sell it to you. Less than $50 if you can install it yourself. The good news is you only have to do it once. If you get it fixed at the dealer, expect to fix it again sometime after about 30K miles. I like the car and want it back, so I hope the dealer will make it right. Mine ended up with coolant throught the engine and it has to be torn completely down. I will try to let you know how it goes.
Doug