On my 90’ 2.3l we have had the same problem. It only happens when the outside air temp drops way off the scale. We fixed the problem with a piece of cardboard. My dad would cover the whole radiator with the card board, and I with ½ to ¾ of coverage. In the hard cold snaps the power plant just would not make enough heat. The cardboard would not cause an over heating problem running around a small town, but a highway trip or towing is a different story. We lived in northern Michigan when we experienced the problem. Now that I live in central Utah I have not ever noticed the problem, but the winters are very mild in comparison. We have replaced the T-stat many times and even replaced the radiator once. Hope it helps, Da’Moose