1.4 Corsa SRI X Reg Engine Problem Help ?

Have a strange problem with my Corsa 1.4SRI 2000 model.
Problem seems to show up at about 45-50mph and feels like the engine is
missing or surging.
Below or above this speed the problem is not there.

I wondered if plugs may be the problem. How easy are they to change on this
model ?
Dont want to use the local dealer due to past bad treatment.

 

does the revs matter or gear?

if it was plugs i would expect it to be not so speed dependent but revs
dependent.
and even then , they would be my last port of call

have you pulled the codes?

mike

 

It does seem to be rev related at about 2500 - 2700.
I suspect the plugs were not changed at the last main service and that was
15k miles back. (Now 58k on the clock).
It does seem to be a narrow rpm band at which the problem shows up.

Any ideas ?