Usual clio problem flat spot, missing underload

[imperial21]

I have a 2004 1.2 16v Clio, D4f engine it starts fine it idles fine but driving it and it has a flat spot around 1500 rpm, I have changed plugs, coil pack, wiring & manifold etc, I am about to do it again as i have another spare one, I am wondering could tappets being wrong and cause it ? I bought the car woth the fault and thought it would be easy, its not throwing fault code up.

I am wondering if its tappets ?
I also know one of the 4 top manifold bolts isnt in as the bit it screws into is missing, I will replace the rocker cover too.

Any more suggestions ????

 

[Typhoon]

Clean up the TB and see if it improves. Belt timing maybe?

 

[imperial21]

I swapped the throttle body with the manifold, the previous garage has changed the head gasket. I will check the timing when apart thanks

 

[Mr Plums]

Maybe an air leak?