1996 Other Suzuki Models Car stoops heating after speed of 5

I have a 1996 4-DR Suzuki Sidekick Sport JLX 1.8 and have replaced two thermostats this week trying to get the vehicle to heat beyond 55 mph. While idle the thermostat registers midway between 'C' and 'overheat'. When driving the faster I go the needle register dwindles down to the 'C' and the heater cools down. The truck does not heat above 55 mph and begins to blow cool air. I can drop my speed below 55mph and the needle will again register between 'C' and 'overheat'. Must I use a specific thermostat? Suggestions please.