Forunner 645 Music Newbie Problems

Hi. I have searched for answers but can find nothing specific to my problems. I have just bought a new 645 Music, charged it and set it up as per instructions. However, it is proving virtually unusable. The buttons sometimes do not work or there is sometimes significant lag and the screens freeze and go dark. I tried to take it on its first run this morning and although it buzzed to show that it had started recording I could oly see the previous screen. It is up to date (6.50) and I have tried turning it off and on again and resetting the setting. Is mine a duff one that I should return or is there a solution?