Same here. Someone ran into my beautiful 91 Deville and totaled it, so I bought this 99 with 60K miles. Took it for short trips and suddenly started to overheat. Took it to Salem Boys and spent $800 on new water pump and thermostat. It still overheated and they tried to sell me a new radiator. News on the web talked about bad head bolts and blown head gaskets, so I did a block test myself and found hydrocarbons in the coolant, blown head gaskets.
There is a possibility that GM will fix it under a recall #99086 that covers vin #XU791685 - XU802142. If GM won't fix it, I will donate it to charity and take my lumps. The Cadi blogs are full of stories of failed attempts to fix the head bolts (that caused the problem in the first place.)
Either way, I'll never buy another Cadi. This is the 3rd one that had had problems.