ASROCK 775Twins HDTV R2.0 Dillemma

This ASROCK model has been my favourite as it has X300 integrated. And I widely use it to build PC’s for my customers.

However, recent builds are almost DOA.

I have found this peculiar problem
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Some can accept only 1 DDR , if u use 2, No display on screen!
Checked both 512 MB ones with memtest – no errors ???

But, if you use 2 X DDR2, gives no problems.
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Anyone got a clue?