1994 Cadillac Deville Coolant?Funny smell/car running rough

Shakes or Wobbles problem
1994 Cadillac Deville Front Wheel Drive Automatic 113840 miles

I bought a 94 Cadillac Deville almost a year ago. Last Feb, the car overheated and was pouring smoke out of the engine- radiator. Since Feb, I have replaced the radiator, multiple hoses, starter, new battery, thermostat twice, had the radiator flushed, power steering pump and had a tune-up with the usual done( spark plugs, etc) The car no longer overheats, the squealing has diminished drastically since the new power steering pump has been put in, but 2 weeks ago, the car leaked out all of it's coolant through the radiator's reservoir tank. I refilled it, and 2 days later, it exploded again. After driving for awhile, the reservoir tank would be boiling, but the car not overheating. The mechanic put a new radiator cap and thermostat in, and the boiling has stopped. But now the car is running really rough, almost shuddering while driving, but has never cut off and there is a sickly sweet smell coming from the engine, but you can only smell is fom outside the car. I've been told head gasket or water pump, but my mechanic says neither, but I don's so much trust him anymore, but really can't afford to go anywhere else. Any ideas of what might be causing car to run so rough and THAT SMELL? Thank you so much, I love this car, but it's beginning to be a headache.