Okay, here's the update: I did the flush, cap and stat today. Because of my own curiosity, and consideration for all of you who have offred thoughts, I did NOT shotgun it and do everything at once.

I did the flush first. During the final rinse, the heat took a noticable jump upward. Back to the point where it is almost uncomfortable to hold your hand over the vents. Roasty heat, or as I describe it to my Chevy driving buddies: FORD heat.

After the rinse cycle, I changed the stat before refilling. On the leak check, the heat performed like it has for the past 11 years- roasty Ford heat.

I haven't driven it down the road yet, so I won't pronounce it cured until I have taken it down the highway, but it appears that the flush made a big difference. I'll report back after I have confirmed whether or not it is fixed.

After driving it down the road, it still cycles hot and cold. It seems as though it gets hotter and colder, too. Hmm.

 

I am having the exact, and I mean the EXACT same problem with my 95 F150 XLT 5.0!
I have flush my system as well! Here is the kicker, I replaced my entire cooling system (All Hoses, Radiator, Heater Core, T-stat, Rad. Cap, H20 Pump) less than 2 years ago. I put on a Heavy Duty radiator. About 9 months ago, I replaced H2O pump again when I replaced timing chain, and cover gaskets when doing oil pan gasket. I replaced the H2O pump with a reman. one. I also replaced Temp. sending unit. The only thing I haven't replaced is the clutch fan. I will just go to elect. fan when my clutch fan goes.
My temp runs fine. I know the t-stat is working, because when the temp starts to go up, it suddenly goes down in the way that is normal for some T-stats when they open.
I flushed and refilled the system about a month ago with one of those prestone flush n fill kits which I have used for years (I love those kits!)
There is no noticable oil in Rad. fluid, or Coolant in oil. I also believe the blend door is working fine, as I hear it when switching temp.
It seems like a lot of people are having the exact same problem!
I'll be following this thread closely! I am stumped!

 

Sounds like an air pocket, the heater core is the high spot in the system(actually the hoses)and its hard to get air out of it. When the truck is cooled off, remove the cap and start the truck. Allow it to warm up, when it gets to temp, if there is an air pocket it will start to spit coolant out of the radiator with the cap off. Either allow it to flow out to remove the air pocket, or accelerate the engine and it will pull the level down and move the air pocket through the system. Finish topping the system off as it is running. Once you think it is full, shut the engine off and let it rest till it is cooled down again, at that point you may need to top it off one more time. The cooling and warming in the cab is the coolant barely flowing through the core with the air pocket, when you first flushed it you got some coolant to the core, but when the rpms went up, you pushed the coolant out, and filled the core with air again. GM's were worse about this, that is why they put air bleeders in their systems. Some Ford motors have them also, but as I recall, they are just on passenger cars, typically 3.8's. Good luck