2000 Mazda Mpv Lx 2.5 6 Cyl

Why Does My Vehicle Over Heat When I Get Up To 70 ? As Long As I Drive 55-60 It Is Fine. What Makes It Over Heat On When I Reach 70 Or More ? I Don’t Have Any Antifreeze In It, Would This Be The Reason It Over Heats ? If This Is Not The Cause Please Let Me Know What The Cause Could Be. “O” I Have Changed The Thermostat Twice And This Did Not Help. What Do You Think Is Causing This ? I Will Be More Than Grateful For Any Ideas Or Help. Thanks .