I agree, the plugs are wires have probably not been replaced in a while. Use only Motorcraft or Autolite plugs and wires.
You say you have had the air filters cleaner? Are you a cotton gauze type air filter? If that is the case, I would strongly consider that you may have a contaminated MAF sensor. Consider switching to a better air filter, such as Volant Powercore or AFE ProDry. They may not flow quite as well as a K&N, but they still flow more air than the engine can use.
I am about 10000 miles away from my next service. Bare with me as I am converting from KM to miles but I had the 100,000KM service done at 120K and will do it again at 220K. that is plugs, transmission kit, coolant.
I will look into a better air filter, I am just using one provided by my mechanic, I beleive it is cotton based.
I also recently had my transmission rebuilt 100%. I lost 1st and 3rd, it is still under warranty but I am not sure if the transmission can affect the fuel economy.
I have 205,000 KM or 130k Miles on it.
Thanks for you input and please keep the suggestions coming....
Also could be a bad thermostat. Same symptoms happened to me a couple of months ago. temp gauge only showed a small increase in temp, but this put the expy into limp home mode and 1500 rpm idle after a short warm up..
OK, I got a call from my mechainc, the engine is running hot at about 105-110. They are going to change the thermostat and perform the cooling system maintenance that I was due to get done in a few months.
I wonder if this would explain why my transmission went at the end of August all of a sudden??