The question does not relate to the B737NG in general, but to specific aircrafts.
Some aircrafts in a fleet seem to maintain the speed better than others do, so this made me wonder if there could be another explanation for this (except the temperature/wind change).
This sounds more like engines approaching major servicing as the turbine blade clearances become wider due to wear and thrust is lost and the FMC expecting more. The more engines are cycled up to high RPM and back the more wear there will be on the turbine blades. Every simple step-climb throttle up reduces the time between major maintenance.