i would suspect deactivation of the PDP Context or a GPRS Detach procedure taking place.
Both should be caused by the core network. However I can’t think of a reason why the MS will re-initiate a GPRS Attach or a PDP Ctxt Activation.
On the radio side, the only scenario i can think of is when the MS is changing Routing Area/Location Area : during the Routing Area Update, it is possible the MS does not display the E/G symbol. The RA takes just few seconds.
Or perhaps the MS was in a PS transfer, and there was a TBF drop. In such case, while the MS is recovering from the drop, it is possible the E/G symbol disappears.
Finally, it’s maybe the phone display itself which doesn’t display the symbol continuously 🙂