solder joint failure analysis

hi, I am a Hardware designer . One of my products is undergoing a Reliability test in which a resistor shows a high value on board from its specified value .However it retains the original value when its cooled under room temperature. The resistor was always subjected to an ambient dry heat temperature of 75 Deg Celcius. The solder joints looks fine and qulifies IPC specs. Can i get some information of 1) Any method to identify the solder quality in terms whether any contamination ,properties changes 2) Any effect of electromigration - the resistor is the series resistor for a pierce crystal configuration

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