Po351 code intermittent..happens below 3k rpm

Question

Hi any help greatly appreciated ,2005 Ford Fusion 1.6 petrol running intermittently on 2 cylinders ,not running on 1 and 4 at tickover and below 3k rpm 
Misfire clears when accelerate over 3k and drives ok all day until rpm drops down to tickover ,misfire reappears after about 1 min and then running on 2   cylinders .   I suspect PCM/ECU is seeing some problem at low rpm and switching off 1 and 4 What I can't understand is why when over 3k car will run perfect..if a PCM/ECU fault surely will fail at any rpm.
OBD reader shows fault code PO351..have changed coilpack twice used genuine ford pack,new spark plugs ht leads,checked connections to sensors and from coilpack back to pcm,reader does not show any sensor fault
Live data check on OBD reader does not show any obvious problem.
Would a PCM develop a fault that only shows below 3000 rpm..surely if an electronic component in PCM goes faulty (if spiked from a faulty coilpack) it will fail at any rpm.

Answer 1

P0351 Ignition coil 1 primary / Secondary circuit

This code it does not have to set a CK ENG LT. because of this this code tends to be a pose a problem , when a vehicle comes into a shop there a customer complaint of a rough idle, without any code set in the ECM , and if a tune up is performed the CK ENG LT comes on.  the real definition is primary circuit low, if you measure the primary circuit of ignition coil 1 and it is below 9.5 volts you need a ECM.