Toyota Yaris Tsport Misfire?

[Mr Gains]

Went down to garage had my car looked at, sounded like a subaru / lawnmower
It also had an engine management light on.

Diagnostics:

INVESTIGATE ENGINE MANAGEMENT LIGHT ON AND POOR VEHICLE MISFIRING WHEN STARTED.
VEHICLE NO ENGINE MANAGEMENT LIGHT ON AND NO FAULT CODES STORED.
CHECKED SPARK FROM PENCIL IGNITION COILS TO PLUGS ALL COILS HEARD WORKING.
REMOVED AND CHECKED SPARK PLUGS, SPARK PLUGS FOUND NOT CRACKED BUT IN POOR CONDITION.
CHECKED COMPRESSION ALL FOUR CYLINDERS HAD GOOD COMPRESSION
CHECKED RESISTANCE THROUGH INJECTORS ALL INJECTORS FOUND BETWEEN 16-16.5 OHMS
FITTED NEW SPARK PLUGS AND ALL COMPONENTS AND RECHECKED WITH NEW SPARK PLUGS.
VEHICLE NOW NOT MISFIRING ROAD TESTED VEHICLE DRIVING AS EXPECTED
SUSPECT SPARK PLUGS CAUSING MISFIRING

I Returned and picked up my car, low and behold it was doing the same thing again. So it wasn't any of the listed suspects.

I have been told it may be the VVTI. What are the chances of this? does anyone know how much it would cost?
Does anyone have any OTHER ideas as to what could be the problem. i don't want to be ripped off here.

Thanks