The code in reply #1 should work but all the variables used in the IRQ should be volatile.
However I would change the order of the statements a bit especially for the var n which is used as an index.
First test if it is out of range before using it.
void DAC_out()
{
conta++; //overflow should not be too much of a problem..in case change to unsigned long..
if (conta >= pitch)
{
conta = 0;
if (n >= steps) n=0;
PORTD=out[n++];
}
}