Is your Modchip a Neo2.2 or Neo4 ?
What DVD game are you using to swap?. As you need a original DVD based game with a TOC size greater than the copy your trying to boot. DMC demo is a good game to use well over 4Gb, also GTA3 as i used to use GTA3 when i had my Neo4 never failed me once. It's sounds to me that this might be the problem. The media your using is fairly decent, try burning them @1X though with primo to see if that helps. Also try Verbatim dvd-r's as they never fail me at all.
Hope this helps!
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