spluttering at steady speeds - no engine light!

[paloma]

My garage are struggling to resolve a suspected misfire, it is only evident at steady speeds. I don't notice it driving around the country lanes, but on dual carriageways and (the now dreaded ) motorways it is gradually getting worse. No engine lights shown.  THere was one which was an O2 sensor, this was replaced, diagnostics cleared down and yet the splutter remains.

First noticed after I had  new tire fitted and I wondered if somehow the 'bespoke' flange that was welded on exhaust system had been knocked somehow --- this has been ruled out, but seems like a strange coincidence?

Any suggestions WHATSOEVER very gratefully received (as I prepare to face the M1 and M25 once more)

 

[Scoobyghost]

How old are the spark plugs?

What code are the plugs?

Have the coil pack/leads been checked and tested?

[Phillip1993]

Was the O2 sensor a genuine 1 as iv had problems with a after market 1