False Overheating Problem

98 Golf Gti - manual - 2.0- 4cyl -no mods
Bought this car a years ago, it's been in the driveway more than it's been on the road. It all started with a siezed water pump. I had it replaced. then the fans stop working - i replaced the fans - the fan control relay quit- the old one fried or something - i replaced it too. - car drove good for a month -came home then the car won't start later that day- batteries were dead ???- recharged batt. - car won't start -wasn't getting fuel - tested fuel pump - pump good -found fuel pump fuse blown- replaced fuse- it blew again, as soon as i started the car - towed it to mechanic - mechanic installed a "hi-amp fuse" that opens contacts when it gets too hot and closes it back when it cools - if i drive the car hard it will open the contacts - but if i take it easy like 50mph or less it will hold - now when i start the car the overheating warning light is flashing - the temp guage never goes past a half. i have no idea what to do know please help.