Just a couple thoughts. I'm also just a mid-level hobbyist, so
someone else might have better input...
First, I'd wonder about the thermostat. They do wear out and
eventually fail, and that would cause your problem. If you have a
Chilton / Haynes book, you CAN replace a thermostat yourself. On this
motor, they're a pain in the !#@%^# to get to, with some hoses in the
way, but if you could replace your radiator hoses, you can do this.
Next, anytime the coolant system is 'compromised' ie loses coolant,
takes in air; you have to 'burp' it, or purge the air. Basically, if /
when coolant leaked, air took it's place. When you refilled the
radiator, an air bubble was 'stuck' somewhere and the net effect is
that you're low on coolant, even after topping it off. Also, when you
tested it, realize that driving on the road, presumably with the A/C
on puts more load on the engine and also generates more heat. To
'burp' it, make sure it's parked completely level, or even with the
nose slightly high, so the radiator cap is the highest point on the
system - air will work its way to a high point. Turn it on, put the
heat on high, and let it run. Check every 3 - 5 minutes, and top off
the coolant when the level drops. I usually do this over about a half
Other thoughts would be the fan clutch (I've had them go bad on two
Fox-body 5.0's), or transmission trouble, if it's an automatic
(although that's a long shot).