vwh2o

I just purchased my 1993 F150 4x4 5.0 automatic with 74,000 miles a couple of weeks ago. It ran great on the 400 mile trip home and for about another 500 miles but, now there is a miss in it. Idles and accelerates smoothly. But when you are cruising and pulling a gradual slope in overdrive some of the valves start to rattle then it starts missing and starts to vibrate badly ( almost feels like a u-joint bad). Kick it down to drive and it seems to smooth out. It started with a slight miss and has continued to get worse. Since the problem started I have replaced the fuel filter, air filter, plugs , wires, cap&rotor and had the fuel injection flushed. Check engine light is not on and produces no codes. Checked the converter with a temp gun after driving about 50 miles today and shows 390 degrees. Pulled the spark plugs out of the drivers side and the 4th plug turned blue like it is burning lean. Anyone else ever had this problem or have any ideas on what it could be ?