We have a bouncing SD traffic following the low availability of MGW.
By SD average duration, do you mean SD mean holding time? This counter also increase when we have congestion.
For example Cell_x: SD traf 17Erl, Avail SD 24, SD block 55%, MHT 9s
And the next day we could get Cell_x: SD traf 6Erl, SD Avail 24, SD block 0%, MHT 3s.