mischl,

@ 20 mhz, loading TMR1 with $63C0 will result in an interrupt frequency of 125 hz.   The program appears to be looking for 25 hz, which is what it would be at 4 mhz.

But the bigger problem is ... With "resume main", you are restarting the loop after every interrupt. If there's anything substantial going on in the main loop, then it may not even make it through before restarting again.

Just use RESUME without the main.
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