Following the thread it sounds like you took the heatsink off and then reused the thermal pad. You can not do that. Thermal pads are a 1 time use. Pads need to be completely (and carefully) removed if you pull the heatsink. Then you need to use new silver based thermal compound any time you remove and reinstall the heatsink.
Some problems you can run into are:
Chip surface not properly cleaned before reapplying the heatsink. A thread of lint or a spec of remaining thermal pad can cause major heat rise.
Putting the heatsink back on backwards, tilted or crooked will cause it to not be in complete contact with the surface.
The fan should be blowing into the heatsink. I've seen them reversed.
The spring clip across the heatsink has an off center V in it. The point of the V should be directly over the chip to make the heatsink seat properly. I've seen the clips on backwards.
On cheap box heatsinks the copper insert may not be lapped flat. if it has machining grooves in it the thermal compound will not be enough to give good heat flow. This is why they use pads on the cheap boxed crap so they don't have to machine it properly. You can lap the copper if it needs it using scotchbrite. The copper should almost be reflective with no sign of grooves when properly lapped.
Any of the above mis-installs can cause physical damage to the CPU so you need to be careful. Remember it's just a peice of glass.
Some people recommend razor blades to clean chips and spread heatsink compound. Bad idea, metal canscrath the surface of the chip. Use plastic Qtips, tooth picks etc..
If a heatsink comes with thermal compound it is usually junk. You want arctic silver brand. Don't put too much or too little on. I'm running a volcano 9 heatsink but that might be a little under powered for an XP2800.