Well I have a 1993 f150 4.9l it has recently started running odd. I do have k&n air filter on it. But I noticed I have a small leak coming from the top of what I believe is the thermostat or the water pump seal? As to what my buddy says and the water that was coming out was a horrible color i will post a picture soon. But it does not seem to heat up super hot on my gauge but seems to be running really hot under the hood. But it seems to be fighting itself when I floor it the rpm gauge will start fluttering up and down and the power does not feel the same. Do you guys have any suggestions or ideas?

 

Here's a picture after I opened the radiator cap this is the color of the water

 

Looks like rust. When's the last time you changed your coolant ? Or are you running water ?
Even in the summer, coolant (anti freeze) works better than water. It also has rust inhibitors which break down after 2 or 3 years, so even if the antifreeze still tests good, it should be changed every few years.
Looks like you need a good cooling system flush, maybe a couple of gaskets and some fresh coolant.