FYI:
I have removed my processor cooler this evening 2 times, cleaned the cpu and cooler with denatured alcohol and re-installed the cpu-cooler again. I have used different amounts of cooling paste, but I can't get the temperatures down. In contrary: the first time I had temperatures which were 4°C higher, the second time the temperatures were more or less as before (1 or 2 degrees higher maybe). So a badly installed cooler doesn't sound very likely.
I've also used the tool IntelBurnTest to test the temperatures, and with the stress level set to "standard" I'm getting temperatures mid 90's degrees Celsius. When I put the stress level to maximum I'm getting temperatures around 100°C and the cpu starts to throttle.