how tight did you go? I tightened mine up slowly going around to each screw giving it a few turns and then onto the next so the pressure applied was as even as possible all the way around.
I did this until the screws started to feel difficult to turn and then I turned them slightly more until the heatsink seemed to be tight enough that it would never move and i could pick the motherboard up by the heatsink with everything feeling really solid.
When you apply your paste because you can not rotate the heatsink 30 degrees each way to spread it you need to use half as much as you normally would say a 2-3mm grain of rice spread it into a 10p sized circle gently with your finger and then put another 2mm of paste in the middle of that cirtcle but let the heatsink flatten it.
I'm getting 27-30c when browsing the net and around 49-52c 100% load on all cores thats with a 4670k though! but at gigabytes auto overclock 4.3ghz settings that used a stupid 1.35 vcore I was getting 60-65c full load
I tightened the screws in turn to try and spread the paste evenly, and everything felt secure at the time!
I've just tried another run of Prime95 for about 40 minutes after manually setting the fans to maximum and leaving the side of the case off, and Core Temp is showing 76-82 degrees but saying it hit a max of 93 while I was out of the room.
At idle, temps are around 30-35, which isn't too far off what you're getting, but clearly something isn't working correctly to keep it all under control under load. I guess re-seating is the next step - any recommendations on what to use to clean off the thermal paste before re-applying?
Also, how much difference would the orientation of the heatsink fan make? I could only seem to mount it with the fan above or below the heatsink due to the brackets restricting the placement, and the fan is currently pointing down which means it isn't lined up with the front and rear case fans. Could that be driving temps up like this?