1998 Mazda 626 engine overheating

Engine Cooling problem
1998 Mazda 626 6 cyl Front Wheel Drive Manual 142XXX miles

I went camping last weekend and my car overheated after 4 hours of driving. It didn't start overheating until I started to hit some hills. At first I thought it was because I had the car completely loaded with camping gear and I probably maxed out the carrying capacity. I pulled over and let the car cool and then I checked the coolant and oil levels. The oil was about 3/4 full so I added some that I had in the trunk for such an occasion. Adding the little bit of oil seemed to improve its drivability because I was able to keep driving as long as I kept my speed and rpm's down while going up the steep hills. I found that when I was going down hills I could go as fast as I wanted but I had to keep the car in gear otherwise the car would begin to overheat. Once again I just figured it was due to the large load on the way up to camping so I test drove it around and it was fine and I got it all the way home without a problem. A week later (today, 7/15/09) the car overheated just while driving an eight mile stretch of highway to my house and I had it towed home. I checked the transmission fluid and it is a coco color (the fluid level was fine though) so I am going to replace it also the engine oil is about 2/3 through its life but not in bad shape. I was wondering if anyone has some ideas as to what the problem could be, other than the transmission fluid.