Edward S - just a thought, sound like engine icing to me not a model problem, roll back and loss of power is symptomatic of restricted airflow into the engines. I am not sure what temps your FSX is giving you dependent on where you are flying but the 727 does not have OAT so take your guide from the ATIS for the airport you depart from. Either way your going to be operating at freezing levels so Do have you engine fuel heat selected before take off?  (see bottom left hand side of flight engineers station, 3 switches with 3 blues lights). You should also check that you have engine inlet heat selected on the upper right overhead panel.

Other than that no reason why you should be not getting proper thrust.