Thanks, but I disabled all the interrupts before
verifying the variable value in main and
nothing happened (besides slowing down the loop).

It seems that the problem is related to the
comparison inside the handler:

if(!(wr == rd))
{ //read }
else { //stop reading}

or with the inc:

rd = (rd + (unsigned int)SIZE);

because the buffer stops reading when the value
of rd is changed to any value different from zero
(wr is different from zero and from rd).

I´ve tried versions with and without casts to
(unsigned int). I can´t see any problems with
the assembly also....;(