I am running out of ideas with this - can anyone offer any advice on what to check next?

The car holds back at light throttle while trying to keep a steady speed, occasionally bringing up fault codes for misfire on all 4 cylinders. I think this is caused by a weak spark or a weak fuel mixture. The plugs have been changed and it is difficult to check plug Kvs with the scope as the COP coils have powerstage integrated. Driving at 50mph, at 3000 revs in 4th gear, the fault is quite pronounced and at that point the fuel trim is still correct and the llambda sensor it still switching. The fault can't be reproduced in the workshop. I have read that the EGR valve is a very common failure so I have blanked it off but there was no change. Feedback from the EGR valve can be seen in live data but I have no way of tell if it's correct. The manifold vacuum seems incorrect at idle, at 250mb. However at idle, again the fuel trim is bang on. I've read some interesting stuff about this engine which runs a higher than normal compression ratio with two spark plugs per cylinder to combat pre-ignition. I have found some corrosion in the 4 rear coils and wondered if a weak spark on the rear 4 plugs would cause this fault. However the coils are £80 each and I am reluctant to change them if there is no definite link.
I would like to fix this as I have seen this fault on other cars, usually MAP sensor controlled, and find them very hard to pinpoint.
Can anyone one help?