1999 Ford Escort Power decrease/hesitation, very strange pr

Engine Performance problem
1999 Ford Escort 4 cyl Front Wheel Drive Automatic 127k miles

Recently had problems with car losing power while driving. Started when summer weather hit (heat). Never did it during the winter. While driving a little bit, maybe a few miles, it would start losing power, but never would die. Car would get slower and slower but every second or so, but it would push forward a little bit, going about 20 mph. After that I changed the fuel filter, plugs, wires, and the catalytic convertor. Ran fine for about the past month. Temperatures outside weren't high though. Within the past week temperatures have risen to 90's/100's and the car has started this behavior again. Car never get's too hot, internally. It has plenty of coolant and never overheats, all other fluids are full as well. After about 11 miles of driving it starts to act up. Vehicle behaves worst when going up an incline/hill. I can barely go over 20 mph when it does this. I test drove it today, and went through backroads with plenty of hills, vehicle performed greatly and ran smooth, I get back to my house and decide to make the run again, with inferior results. Car slowly crawling up the hills I had cruised over only 10 minutes prior. Best part, if I turn the car off, and turn it right back on, it runs like a champ. Today when I made my second run, when it was losing power on this steep hill, I put it in park, turned it off, turned it back on, and drove right up it without incident. Car is hesitant on flat roads as well, but I can maintain 40-50 mph, but I can feel it hesitating ever other second. Also the mpg I get it crazy with this problem. Last few weeks, I've gotten pretty good gas mileage. When I fill my tank up, the arrow goes a little bit above the full mark. I have been making 100 miles easily before the arrow reaches the full mark. Now with this problem, I filled up the tank. It read above full like usual, and I've driven 94 miles and I'm at an insane 3/4 tank!