What van cause 91 voyager to run hot one minute and 1/2 the next.?

voyager runs hot one minute and the next it cools down to the 1/2 point and stays there and one trip on highway long distance it stayed at the 1/2 point and the next trip different day it got 3/4 the way hot and then went back down to 1/2 and satyed the rest of the way home. We have already replaced the thermostat and the fan is working and we have flushed the radiater and it does not leak any fluid. The lifters are tapping a little and it does need a starter and a new muffler but it has power and overall wuite motor what should I do