When I had my Sportage I always used factory Kia thermasats, when I tried a parts store one it never worked right. The problem you first described sure sounds like a thermasat though.

95 Sportage, gone but not forgotten.
98 Jeep TJ, 4" Zone springs with RC Long arms, Ford 8.8", 4.56 gears & Detroit locker rear, D30, 4.56 & Detroit Trutrack front.