weird that on first switch on , the broadband speed shows 600kbps but on reboot of modem, it gives 10mbps

Hi, it is a weird problem, as i do not have any problem with signals.
on first booting of modem, it shows a very low speed, so that even all india radio stations could not play in pc and in mobile thro wifi.
But when i reboot, i get the normal speed allotted to me.
is there a fix, what could i do to analyse this weird problem

Specify your router brand . Also attach the pictures of your router configuration