Overheating/ Help? Suggestions?

[jcoogs]

2001 Alero, 3.4 6 cyl. 210,000 miles. A few times over the past few weeks, the temperature gauge went a little above halfway and then dropped back down to normal. Then last week the car started overheating. I thought I had the solution, as I had the same problem both 10 and 5 years ago, which was solved with a new temperature sensor and thermostat respectively. I replaced both this time, BUT as I poured new coolant into the reservoir (with the car off) I noticed it spilling down onto the ground, originating from somewhere near the thermostat housing (but not from the thermostat housing- that's dry). So, I'm guessing I have a crack in my engine block in some inaccessible place that I wouldn't even be able to see unless I lift out the engine. Or could it be a bad head gasket? In either case I'm guessing that would indicate a very expensive and/or futile problem, eh?