What is this "knocking"?

We have a 2002 Explorer XLT with 8 cylinder enging, rear wheel 
drive only, 53000 miles.

Have always had it serviced by the dealer.

Some time ago it started "missing" or what ever it is. This is 
very erratic. Seems to happen most when doing 40 - 50 mph 
and I slightly push on the gas pedal. It even happens with the 
cruise control on and we start up a hill or bridge, etc.

I can "stomp" on it and nothing strange happens.

I have asked the dealer to check it out.

They did some "tests" and "reset the PCM". Six or so days later 
it was back to the same old thing.

I did discover that putting a container of Techron in the tank 
just before filling up completely stops this problem.

Chevron gas which contains Techron does not stop it.

I mentioned cleaning the injectors or replacing the spark plug 
wires. They weren't interested in eith