Not quite sure what measuring the temp - off CPU - with an infrared thermo achieves especially when everyone has different hardware on the cooling loop and differing ambients. Especially as -15ÂºC and a Dt >40ÂºC would be quite reasonable to expect with roughly 75% power. One needs the temp measured in situ on the CPU - which, of course, your not kitted out to measure.
Incidently what are you are refering to as ambient - room ambient or case ambient ?
Depending on the quality of your case cooling your case ambient can be several degress up which in turn affects your hotside temp. ( and works against the cooling provided by the rad.) and since your Dt is subtracted from the hotside temp to arrive at your coldside temp (i.e. your hotside cooling is paramount.) it may be worth investigating your case cooling/airflow (if it's not very good.) to get better cold readings.
Edited by zipdogso - 4/12/10 at 7:22pm