in general cases it would be the heat sink is not well secured .. so after a while of using the thermal paste run off of its default position due to fan vibration..

or there's possibility that ur heat sink is not flat surface maybe old thermal paste still around ... .. . do make sure u clean both sides nicely .... not even any stain of thermal paste left ..

if both also doesn't works u might need to snap some photo of ur rig and heat sink ...

ohh.. dear without the thermal paste the temperature difference would be as much as 5 for idle and 15 for load... of course it is not ok. never trust the guys from the shop when u aren't familiar with them ...

they just simply ignore newbie ... coz from taking off to put back the heat sink , there's more steps and it is troublesome to describe .. so cut to short 'No need to apply....'.....

I'm not sure what's the hole on the CPU for either even though i know it's there when i was still using the P4 series ... actually u could just use a tooth pick and dig out the accumulated thermal paste....

afterward just apply another thin coat of thermal paste... there are two methods to get it applied.

1. squid a certain amount into the center of the CPU then just install back the heat sink. Let the heat sink do the spreading job...

2. Squid small amount of thermal paste, use a plastic card such as reload coupon. Spread it evenly with the card in horizontal and vertical direction.
Fill up the area that's lack of thermal paste carefully. It is a tougher job but it ensure ur thermal paste is well spread and u have full control of the thickness as well.

anyway it's not necessary to get arctic cleaner .. any tape cleaning alcohol well do as well. besides, to clean the very old thermal paste, use the cotton stick and spread the alcohol on the surface. Let it be for few seconds and use the stick to remove the harden thermal paste..... afterward get a piece of makeup cotton mix with alcohol and wipe off the remaining.