EGPRS(EDGE) data rate is not stable?

With phone attached to my configured EGPRS, sometimes it has a data rate around 200kbps (which is near optimal), while frequently it has a intolerablly slow data rate. Is it a common problem, or it's due to incorrect configrations or hardware problems? I run a gsm test system with nanobts, and the libosmocore, openbsc, openggsn codes are based on the version around end of April 2015. The configuration is made accorrding to the instruction from openbsc web. The correction in fc_queue_timer_cfg has also been made according to Jacob Erlbeck.