I've had my 2004 ST170 for 2 years now, and I love it to bits...which is quite wierd since I've owned my 2007 Wolf ST3 ASBO for over 5 years...and really prefer to drive the 170 on a daily basis.
...But there's a problem I just can't get my head around.
The engine performs perfectly (with the usual intermittant hesitation when accelerating through 1500 - 2000rpm), as expected with Ford, all the way up to 6000rpm...as long as I stay in 4th. As soon as I select 5th/6th, I get a misfire around 70mph, and insufficient power to accelerate. Like I said, if I drop to 4th & boot it, everything is fine...all the way up to 6000rpm.
It's running fully synthetic 5w30, changed in early November.
I also changed the plugs, as a matter of course...but there was no change.

Can anybody shine any light on this?
Maybe a swirl flap, cam timing issue???

Thanks in advance