Please help with MA78G-DS3H ridiculous system temp reading

Hi all.
This thing has really pi**ed me off.
To cut a long story short, I assembled my system with MA78G-DS3H in late August 2008. I am using the onboard video.
I use Everest Ultimate to monitor the temp readings, BIOS displays CPU and System temp while Everest offers Motherboard (guess it's same as System), CPU, both cores and HDD.
The CPU temp was always good in the mid 30es C. Motherboard temp was few degrees higher.
Few months back I replaced the 80mm fan on the back of my case because the manufacturer supplied was noisy. Even after that my Motherboard temp was ok, low 40es. And I clearly remember the new fan working only on 800-900rpm (max is 1500) because I have it plugged in SYS_FAN1 which offers speed reduction according to motherboard temp reading.
Everything looked fine and I wasn't checking Everest for a long time. Few weeks ago I started Everest and was shocked (literary) to see motherboard temp at 77C??? Worried that this might damage my northbridge (earlier I was told by GB support that the second sensor is close to the NB and motherboard/system temp reading can be considered as NB temp) I started reading how to bring down this temp.
First I checked the BIOS too, and the reading of 77C was confirmed there so it doesn't look like Everest mistake.
Then I opened the case with the PC already running for hours and put my finger on the NB heatsink. The heatsink was barely warm. I can not tell exact temp of course, but I could hold my finger on it quite long which I doubt I would be able to do with 77C?
To go one step further, today I purchased 40x40x10mm fan and attached it on top of the heatsink with two screws and 8mm plastic spacers to give it more room. The fan is blowing air downwards towards the heatsink.
Guess what, Everest still says 77C (in fact 78C right now)...????
I mean, there is NO WAY the NB heatsink temp is 77C right now with the 40mm fan blowing on it. Because of the desing of the heatsink (there is a higher and lower part) I was still able to put my finger on the lower part of the heatsink and it is literary cold not even slightly warm (which I would expect with the fan blowing on it).
I was trying to avoid buying a temp sensor just for one test measurment but it seems there is no other way to know if those 77C are correct. Even before attaching the fan I was wondering if I could hold my finger on 77C heatsink but now with the fan attached there is no way I didn't gain at least few degrees.
I admit I do not have decent air flow in my case, no front fan and only one fan on the back blowing outwards. I will by a 120mm for the front these days. But even with the current situation the NB heatsink should not be 77C and it seems it isn't.
Do I have a bad sensor? Am I freaking out over nothing?
I can not return the board and turn out that the sensor is right. Even if it is wrong I am wondering if returning the board is worth it because I will be for weeks without the PC.
Any ideas???? Sorry for the long post but I tried to explain everything I did so it could help you.
Thanks in advance for any help. Cheers.

Thanks for the reply.
Earlier I had GB support confirm that mobo/system sensor is close to the NB and you can consider system temp to be NB temp. That is as long as you can trust their support. I haven't been able to find precise information where that sensor is otherwise.
Assuming the HS has come separated from the chip, can I do a quick test by pressing it gently to see if something will change once it comes in contact with the chip again? I am trying to avoid removing my board just to find out it wasn't the problem. I would have to remove the board to replace the HS. Is Zalman NB47J good for replacement?
I was considering front intake fan in desperate circumstances. My PC is just ordinary home PC, no gaming, no video rendering, etc. It doesn't even need some great air flow. I had a second exhaust fan on the side of the case but temporarily removed it because these days I am opening the case frequently with this 77C 'problem'.
One question though, I have Asus TA-863 case with place for two exhaust fans on the back. However, the holes are so close that the two fan I have, silent Coolink and Coolermaster when installed with screws are so tight to each other the one of the fans doesn't work. The body of the fan is actually pressed on the fins and preventing them moving. If I install both fans with those silicon gel mounts, it would give me slight flexibility, can I expect both of the to be working next to each other? I am planning to get a pack of those mounts soon.
Thanks again. Cheers.

like mondoman said, the heatsink might have come unstuck. tyr aplying pressure to the top of the HS with your finger. this might increase contact area, thereby letting it cool down. hold your finger there for a while and see if the temp goes down.

if it has stayed within 1 degree of 77C regardless of what you have done and the heatsink doesn't wiggle around if you touch it, then it is safe to say your sensor is the problem.

just keep using it normally, a good sign that it is a stuck sensor is if you turn your computer on after hours of it being off and looking at its temp through the bios. if it displays 77 then it has to be stuck because it would have had no time to heat up. after a while your sensor may self correct.

I agree with cheep, seems like a bad sensor.

As they say in court: "already asked and answered"