Connects on wifi problems with 3g??

Hi i have installed free version, My problem is it connects fine through home wifi network display perfect - but when i connect through 3g it connects but screen goes black and stays that way till i disconnect.??... bugging me for 2 days now also have all updated versions -

phone - Xperia x8
OS - windows 7 premium

also have tight vnc 2.0.2 latest version..

Many Thanks if anyone can help me.....