Generating 3-phase sine wave..

[skimask]

Sylvio,
This may sound silly, but are you sure it's a 20mhz crystal?
A 4mhz crystal with that Timer1 reload value would give ~870 hz.
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For that matter, it might be running on the internal 4Mhz clock and ignoring the external completely.
Pull the crystal out and see if it still runs at all...