Coolant Loss / Headgasket

I have a '98 Cadillac Deville; 187,266 miles; Northstar 4.6 engine; 2 wd.

For some time, I've had a problem with coolant loss. Upon taking it to the shop, I've had the following things done. Heater core, raqdiator, water pump, thermostat, hoses, drivebelt and tensioner for the water pump, and main drive plus accessories. Recently the car began running hot, bubbling from the over flow tube. While changing the starter on today, and rechecking all connections, I noticed air/water bubbles at the rear cylinder head. I am assuming that I am dealing with a blown head gasket. Am I correct? This car has been really good to me, and I've kept it serviced on a regular basis. Any advice would be helpful.