Overheating P4 problem

Got a Foxconn 945G7ad for 18 bucks a couple of weeks ago. Thought I'd match it up with a P4 2.8gig I have until I find a good deal on a E6xxx. It's mobo died some years back and I thought this would make a good first project to do with my boys. And it was.

After we got it up and running I noticed the temp at idle sat around 40 Celsius but then it would spike up to 56 and even 60 with a light load. I'm using SpeedFan 4.39 to monitor the temp. So I downloaded Orthos and stressed tested the CPU. Sure enough within 2 min. the temp was in the 69-72 range but would drop down to the mid 50's every 20 sec. or so. Then back up again. Afraid that I was going to hear the mad beeping of an overheat warning from the mobo I turned a 12 inch fan on the already open case. This brought the temp down to a consistant 65-68 temp range without the 15+ drop I was seeing every 20 sec. or so.

I bought the CPU bulk(Fry's mobo and CPU special) so I got an oversized no name aluminum heatsink with a 90mm fan that turns at 2800rpm. The sink has a copper center that makes contact with the top of the CPU. When I had the pair installed on the old mobo I never saw the temp go above 56. Can heatsinks go bad?

The paste is called KPT-8 bought in Russia. I put it on nice and think. At least 2mm. I know it's not a bad performer because I replaced the thermotape between a 65w X2 3800+ and it's stock heatsink with this stuff and saw a consistent 4 degree drop in temp. after.

So I don't know what to do. Leaving the case open with a 12 fan blowing on it is not going to fly with the missus. I know the P4 is high watt and so high heat. But I don't think this is the kind of temps Intel designed this for. I'm guessing mid 50's as a high is suppose to be normal.

Any ideas? Maybe the Intel style push pins(don't know how you call them) get old and stretched and they are not holding the heatsink tight enough to the CPU?

Could it be the CPU became damaged somehow when the old mobo gave up the ghost? But if that's true why is it working at all? There we no errors or warnings during the 45+ min. I ran orthos.

So thanks in advance for any ideas you can throw my way.

Thanks GhislainG. Your right I put too much. Some great vids on youtube about applying paste to your CPU.