i’m a little confused when you say “pdp drop”. You mean the PDP is deactivated ? by the GGSN ?
And then a new pdp context is established when MS enters the 2G network ?
From radio side, there is nothing that would impact the PDP. I would focus entirely on the GGSN settings, and possibly check behaviour of MS. Detect who is dropping what, find this scenario in the 3GPP docs. And from there, I think we’ll find something 🙂