2012 cb1000 r overheating

Hey guys please I need your advice.

I’m facing an overheating challenge with my 2008 cb1000r.
I noticed it three weeks ago but it’s cool on the open road and overheat in traffic or on a slow cruise.

Fan and temperature sensor are perfectly fine
I have washed the radiator
Replaced the blown water host
Add seal on the radiator cap
Flushed the coolant and running with ordinary water
Current temperature is Sierra Leone is 29 degrees.
still facing overheating.

we actually do not have professional and good Mechanics in Sierra Leone.
I’m suspecting the cylinder head gasket but I don’t want purchase and open the engine and still face same thing after.

so please I need your advice.
I really love that bike and want to get back on the road.