This maybe a stupid idea from me, but if you try using >= instead of ==. I'm thinking maybe your program is not checking the if statements at the exact time that they are true. Just a quick thought between bites of my lunch.

That wasn't a stupid idea at all - so I implemented it, to no avail :/

Meanwhile, timer-init is back to this setup:

    EA = 0;                        /* disable interrupts */
    TMOD = (TMOD & 0xF0) | 0x01;   /* Set T/C0 Mode */
    ET0 = 1;                       /* Enable Timer 0 Interrupts */
    TR0 = 1;                       /* Start Timer 0 Running */
    EA = 1;                        /* enable all interrupts */

And I added a pin-toggle to the interrupt-routine as well one as to the main-routine, just before the if-statements. The interrupt is triggered approx. 25 times per second, the main-routine quite a lot more often (my poor old scope was almost at the end of its capabilities ...).

/Thomas