OK I've finished reapplying the artic silver to the CPU heatsink
Did you verify the contact after applying the arctic silver? I would check that the heatsink is making full contact with the processor first.

I usually spread an even coat on the cpu and then put the heatsink down on it. Pull off the heatsink and look for gaps or unevenness. If the heatsink isn't absolutely flush with the cpu, that's the problem. I know it's a laptop, but I've seen where heatpipes get bent and slightly pull the heatsink off once everything is installed.