OK you should have a big delay, that indicates you need to start counting. Then 13 flashes, whatever flash is slightly longer is the problem. So i'm guessing you're seeing flash number one as more delayed (ie big delay, then flash one delayed?)
Which is Vehicle Speed Sensor 1 (Output shaft speed sensor) it's the sensor on the gearbox visible from underneath on the drivers side, orange/brown plug I think.
May be something as simple as needing the contacts cleaned, seeing as it's random (and something they may have touched fitting the shift kit)
I'd unplug, clean and clip back on.