Squawkbox wind problem

Hi I have a problem with Squawkbox and winds: I can be flying along in my B738 at FL370. Pressing shift-z shows me an indicated airspeed of 270kts and the GPs shows a ground speed of 458kts. So i think all is normal. Then the controller comes to me and asks me to confirm speed because they are getting a reading of 120kts ground speed. So i look at my squawkbox window (with the TAS and IAS in it) I can't remember what the TAS was but the IAS was varying between 105kts and 280kts. And the SB wind roud out was at times up to 300kts headwind. A few minutes ago i thought it would try and fly an orbit to see if i ended up with a 300kt tail wind. But no, the headwind remained for the full 360 degree turn. I have spoken to numerous people and no one seems to be able to help me. So the controller sees my ground speed as it is indicated in SB and yet in my GPS i see a ground speed of over 400kts. Any ideas on how i can solve this? Fernando Cobos