2000 Nissan Xterra Overheating

I have a 2000 Nissan Xterra SE, 4WD, 6 cylidenr with 99,000 miles on it with a severe overheating problem. For the past two months my car has been overheating constantly. I took it to nissan and they told me it was it was a bad intake manifold gasket. A few hundred dollars later, I took it home only to find out it overheated again. I went back to nissan and this time they suggested replacing the hoses because sometimes they can create this type of problem. Did that. No change. I then took my car to another place where they changed the thermostat and checked my radiatior. I was told the radiator was fine and hopefully it would stop overheating. No luck. So I took it back to nissan where they said maybe replacing the water pump would help since there are gaskets there that could leak and cause the car to overheat. They also said it would cost up to $1000 dollars to do. Well after hearing that and the "maybe" part I decided to try a completely different mechanic.
The next mechanic said that such a small leak if there was one in the water pump would not cause such drastic overheating right away and when he looked at my water pump said that water was moving through the system and that it was probably something else. Two days and several hundred dollars later the fan clutch was replaced hopefully solving the problem. Wrong again. Ten minutes later the car overheated. The car went back today for another check and the mechanic is now telling me "maybe" its the head gasket and replacing it will solve my problem. Not only did he tell me it would set me back almost 2K but when I asked another mechanic for an opinion they told me that would just ruin all of my other gaskets and I would end up paying more in a few months time to fix them too. I am in desperate need of advice. The only problem I had right before this is that the heater core broke sending coolant through my air vents when the heat was on. I had someone replace this for me and now ironically the car won't stop overheating. I thought maybe it could be the radiator or something obvious since it overheats every time but my mechanic tells me its fine. I need help. I can't afford to pay any more and not have a definite fix. Does this sound familiar to anyone. What should I do? :Cry: