Engine Miss

[symonab]

I was going to entitle this thread as missfire but that isnt correct.

Background:
V70R AWD
Previous owner NOT a petrolhead nor had a passion about volvos

I do therefore:
ECU upgrade by Don @ Kalmar (problem evident before upgrade)
Ferrita Twin Exhaust (only beacuse original rusted?!)
running genuine components and all ignition system recently replaced from coil down.
Drilled air box
etc etc

Problem - reset the ECU and the car drives perfectly for aprc 250 miles. Then under low throttle in 3rd or 4th a slight blip between 2k & 3k rpm almost un noticable. Then again after aprx 75 - 100 miles, then again between 25 & 50 miles.

There isnt a back fire - I initially thought that I was lossing spark but that may cause a slight missfire. Therefore replaced all inginition components. I cannot believe that it is fuel pump related as all the signs point away from this ie not happening under full requirement of fuel and I can now accurately estimate when the problem is going to occur after resetting ECU.

Therefore the question. What inputs go into the ECU that would tell the coil not to spark. I am starting to suspect a sensor now - pos lambda or foot throttle position.

Any pointers or ideas would be appreciated. Its not the end of the earth but a little annoying.

Cheers Sy