Car will not start when hot

First thing in the morning car starts and runs, and reaches its average heat range. Once I turn it off, and try to restart (after a few minutes), it will still restart. If I wait an hour or longer, it will not restart on its own. I have gotten it restarted with starting fluid, but it can take one to many many tries. During the winter months, it cools down fast enough to restart (sometimes with fluid). Now that it is spring, it is taking longer to cool down enough to restart (even with fluid). The twist is, if I let it idle, it will heat up to about half on the temperature gauge. Once I shut the car off, it seems that if it has not cooled down to about 3/8 to less than 1/4 on the gauge, it will restart fairly well. Once it gets down to the average range, I have to use fluid or wait till it cools down completely.