Overheating/Oil Problem

[Erianc]

I have a problem with my 04 5.7. Just the other day, I was on my way to school in the morning and had the “Check Gauges” symbol pop up. Truck was overheating so I pulled over and the gauge went back to normal temp in less than a minute. I kept driving and the gauge would stay at 3/4 of the way. If I stop at a light, stop sign, or just park, the gauge will drop back down to under halfway in about 30 seconds. The trucks been running fine and has given me no recent problems before any of this overheating started. In a matter of a few months I’ve replaced the water pump, thermostat, upper and lower rad hoses, new fan clutch, did a coolant change and an oil change, so it’s weird to me for it be overheating. Started to think it was a head gasket problem or internal crack, so just today I took out spark plugs 1 and 3 and there was oil on the threads. No oil or coolant in the seats, though. Also noticed that I lost quite a bit of coolant through just driving the truck home the day of and running it to see if it’d overheat again, thinking there was an air bubble. I checked the oil cap and there was no white residue on it. However, there is oil collecting under the truck around the oil pan. I drained some oil and my most recent oil change was three weeks ago and although there was no coolant in the oil, it was real dark. Even held it up to a light and there was no light seeming to pass through. If anyone knows what the problem is or might have experienced something like this before, any advice would be greatly appreciated!