External temperature sensor

I have a problem with a board that hold seven TPS544C20 they generate the core voltage of seven DSP 6670. In order to track precisely the board temperature i have connected the external temperature sensor (as suggested an MMBT3904) to the TPS544C20 but we are not able to read a stable temperature, it changes tens of degree in few minutes also if filtered via software by an moving average.
The sensors read a value that in many case vary each to other of about 20 degree celsius, that isn't real (also if placed in different position ), other non TPS544C20 temperature sensors on board report a stable 37 temperature degree. The Vbe measured by a voltmeter shown a stable tension that change in function of the temperature, using a oscilloscope we have discovered a 50 mV periodic variation of the Vbe (about every 8mS for a millisecond) that seems related the temperature acquisition is that correct ?
May this periodic variation be the cause of the unusable temperature readout ?
PS: the layout is good and the traces are short !
thanx
Maxx