I have a acura 1997 3.2 tl, it over heats

the first time I drove about 60 miles , I added water it stopped, I droved it
back home and it was fine it didnt do it on the city, I took it to the mechanic
he replace the thermostat and it was fine on the city for acouple of weeks, I
drove it on the high way about 60 miles and it did it again, I took it back to
the mechanic and had him remove the thermostat it drove fine untill next
time I went on the high way, this time I drove it 260 miles before it heat it up,
I took it to an other mechanic in the town I was in he ran it for 2 hours with air
on and off, and couldent get it to heat up, he cleaned the exterior of the
radiator and I drove back home it heated up after I drove it like 250 miles, I
cooled it off and drove it home and it didnt heat up, all this high way miles
where on themperatures of A 100 dg plus