interference proof voltage regulated current source
First you should monitor the current with an oscilloscope. The circuit you have is prone to oscillation, and that's probably what's happening to make the LEDs go bright. You should stabilize the circuit. I'll add a schematic in an edit in a bit. 

Secondly, you should check the voltage at pin 3 of the op-amp- you've probably got a firmware problem of some kind that is affecting the PWM. 

Edit: If you look at the datasheet, you'll see that loads in excess of 100pF can negatively affect stability. Obviously the MOSFET gate is more than 100pF, and the 2.34 ohm resistor is almost a short. 

So, here is a way to deal with that: 







simulate this circuit &ndash; Schematic created using CircuitLab