EPDM - A marching modulus can be seen when the sample is inadequately cured, which can be remedied by increasing the cure temperature. 

 Reversion happens due to degradation and side reactions that occur in the rubber system.  A plateau modulus is the desired outcome due to curing being complete and an absence of degradation .

For more details pls refer "The Curing and Degradation Kinetics of Sulfur Cured EPDM Rubber" A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science By ROBERT J. WEHRLE B.S., Northern Kentucky University,

I agree with Sasidharan also a marching cure is a common characteristics in peroxide type cure due to incomplete cure in EPDM. In sulfur cure plateau modulus is achieved quite conveniently. For reveresion, you may use access amount of sulfur or co-cure accelerators. For example try following formulation

EPDM 100 PHR

Stearic acid 1

PEG 3350 2

ZnO2 5

Sulfur 3

ZBDC 2

TMTD 1

MBTS 2