Thanks fellow riders.
will visit my mech's shop this friday and will tell him to inspect all these.
the vents are not obstructed. no cleaning cloths there.
vapor lock and/or blocked coolant passage seems to be the potential reason. the fan was working fine for last 3 months, all of a sudden this fan going weak seems unlikely, still, nevertheless, will check that too.
let's see what happens, will keep you posted.