Windstar intermittant rough idle

I have a Windstar '98, 168.000 miles, 3.8.   It has started to idle very
roughly, but the problem is very intermittant.    It seems to only occur
when sitting in traffic, idling, in gear for over 5 minutes.   The check
engine light does not come on, even when it is obviously misfiring.

I had the computer scanned and no codes were stored.   I'm a little
confused, as I thought the computer would pick up any type of misfire, so it
could be diagnosed.  I was hoping it would at least tell me which cylinder
had the problem! Any ideas as to why the computer does not give me any
informaiton?

I talked to the dealership mechanic and  and he suggested I start by
replacing all the fuel injectors.  He said as the FI ages, the resistance
goes up when the FI gets warm/hot and are a very common cause of
intermittant misfire,  especially in the conditions I describe where the
under hood temperature goes up from stop and go traffic.

He also told me 150K is beyond the service expectation of a ford fuel
injector. He also recommended buy the full set of FIs, as they are usually
within 3% of each other for fuel flow.  As if you buy them one by one the
fuel flow could vey by as much as 15%.

This sound like an expensive fix for guessing what the issue is.  Any
suggestions?