It maybe a bad battery pack or a bad screen? I assume you mean the screen is flashing?
http://www.google.com.au/search?hl=en&safe=off&q=my+nintendo+ds+keeps+flashing&meta=
http://answers.yahoo.com/search/search_result;_ylt=Alqle08MnFR49SUDEgCwwnF7.Bd.;_ylv=3?p=ds+keeps+flashing
Buy a replacement screen or screens or battery pack on ebay or try one of the DS forums that are in plentiful supply on the internet. Google for Nintendo DS forums and search for your problem.
Parts:
Assuming your are in the USA try here:
http://www.consoleshop.com/home.php?cat=303
http://powergamingparts.com/nintendo-ds.html
In the UK:
http://www.ps2laser.co.uk/ds-lite-repair-parts-spares-accessories/DS%20Lite%20parts/c/118/143
A quick search of google shows:
http://www.google.com.au/search?hl=en&safe=off&q=ds+parts+usa&meta=
Try one of the dealers on ebaY:
http://shop.ebay.com.au/?_from=R40&_trksid=m38&_nkw=ds+parts&_sacat=See-All-Categories
Steve, June 2009
