Hi! I have a 1999 Ford F-150 with 203,000 miles manual 5-speed transmission 4.6L V8 triton engine, with a K and M cold air intake . When driving and the truck reaches higher temperatures, driving for about 1.5 hours in the summer, or 7-8 hours in the winter; some symptoms occur.

The symptoms are: the truck has a really rough idle, usually dying. Sometimes, especially at lower speeds, the accelerator peddle will randomly not have any power to it. I will press on the peddle and nothing happens. I'll have it floored and then all of a sudden it will have a clunking noise from the read end and the peddle will have power again, and shoot forward. When the peddle regains power it usually lurches quite a bit. 

This has been happening for years, and never tripped a code before, until recently. The code that was tripped was P0174 Bank 2 system too lean. My first thought with the heat symptom was a vacuum line leak. I had a mechanic run a smoke test. The only thing that showed up was gas tank Evap valve was not sealing properly. The clunking noise is from about the right area to be the Evap valve, but is hard to say for sure. Could this be the issue? 

I have already cleaned and tested the EGR valve, cleaned and oiled the cold air intake filter, cleaned the MAF sensor, ran Lucas oil fuel system cleaner, and tested the PVC valve (which is on the bank 1 side anyways). 

Thanks in advance!