Dell DIMM 8400 heatsink not seating

My troubles began when cpu would only boot to a blinking cursor.  The diag lights AB - Green CD -Amber  Which told me a pci card needed to be reseated,  I took the tower off the desk and layed it down on it's side.  I opened the case & took out the parralel port and rebooted the cpu and it worked great.  I decided to reseat ALL the cards and then rebooted again - all good.  I closed the case and set it upright on the desk and it would only boot to a cursor.

I took it back down and layed it on it's side and tried all the cards (except video) out - blinking cursor. Then I put them all back in - blinking cursor. took one out at a time - blinking cursor.

The next day I started the pc and the fan roared like a jet.  No diag lights no blinking cursor.  I was advised that I should make sure the clips were not broken that hold in the heatsink and the plastic rails were not melted.  I took the heatsink out, and everything looked in order.  I replaced the heat grease - put too much on so had to take heatsink back off and clean the grease up and put a lighter coat on.

IF I PUT MY THUMB ON TOP OF THE HEATSINK AND FIRMLY PUSH DOWN THE SYSTEM BOOTS NORMALLY AND WILL EVEN OPERATE PROGRAMS.  But the min I take my thumb away the screen freezes.  I say an article where someone else had the same issue and they used rubber bands to hold the heatsink down.

I would rather fix it the right way, BUT have no clue as to how to fix.  (I also unhooked the cpucover and lifted it up and down --- NEVER lifting the cpu.  And yes I did re-hook the cpu cover so it is tight.

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