My sisters partner has a 54 reg Renault Laguna 2.0 Petrol Automatic with 84K miles on the clock, the car has a strange fault which I cannot diagnose.

 

Car stars fine and revs as you would expect, no sign of hesitation or lumpy idle, but if you go out for a drive at say 60mph then the car feels almost like its misfiring, happens on constant throttle and if you speed up. The car never stalls, it just runs weird. I don't think its plugs, in that I can rev it to the red line whilst stationary and its fine. He's driven it like this for ages, there are no engine fault codes or anything untoward on the dash.

 

Thoughts anybody, do these have EGR valves for example.

 

The car is currently outside my house in Blandford, so I can try a few things if need be.

I would try a bit of carb cleaner on the throttle housing first, could just be gunk on the butterfly. My petrol Octavia used to suffer terrible from this.

It could still be bad plugs; I used to drive a Cortina that would redline light, but started misfiring at 4_000 under load. The cause was cooked plugs and the cure was changing from iron core to copper core.