Ignition after big service?

I've had a big service done at 130000 km, on my Civic 1.5iLS and
I had timing belt replaced, spark plugs replaced, ... injector cleansed
(ultrasound).

Now my car seems to ignite strangely, as if it starts to run imediately
after turning the key
but irregularly (like not all cylinders work), or it even doesn't start at
first, but when engine
is warm it has Honda characteristic ignition sound and it turns and ignites
as it has done
previously to service.

Why is it igniting strangely when cold could it be some timing problem
because timing
belt was replaced?
Could it be the injectors (one injector seemed to be not working like other
three)?
Could it be bad gasoline or cloged gasoline filter?