This is normal behaviour I believe. While there is no data transfer the signal will show 3G and while data transfer it will switch to 3.5G. This does mean that you are getting 3.5G signal all along. You cant just force it to show 3.5G always if there is no transfer going along. Relax the 3G-3.5G switching denotes 3.5G reception. Only in case of 3G-2.5G switching denotes bad 3G reception.