does it happen at full power? Sorry - I jumped too soon - - I think the value of R1 is too low. Increase the pot [ 20K s/b ok ] and R1. The "upper side" of the pot and R1 are setting the current - you have too much current flowing when at or near full power. Q1 needs too much base current for the pot.
But after increasing the pot and R1, Q1 may not be able to drive the LED to full power.
then you have to;
- use a 2n3055 with higher gain; hfe
or
- use a darlington for Q1
or
- use a different Q1 with higher gain
R1 does not have to be 5W.
It would be better [less power wasted] to control it using PWM. A simple 555 based circuit can have a duty cycle from 50% to 99%.
A little more complex 555 circuit has a broader duty cycle.
Or you could control the current rather than the voltage.