In all such programs where one would like to change values on the fly, some discipline has to be there as to when the change can take place.

Assuming that a particular value of P2 , say 200 is timing out and you change the value of P2 before the current counter loop has terminated, will lead to problems. If I were you, I would only check P2 at the end of one complete loop.

Maybe in the ISR after the "counter=0;" statement you can load P2 on to another int variable and use THIS variable as the loop counter.

Raghu