Screen Anomaly

Hi guys,

Seeking opinions/insight regarding a screen issue with a 15" 2007 MBP A1226.
Internal screen shows desktop with all artifacts but is laced with numerous horizontal lines. External screen is fine with a bright clear desktop. Internal screen used is a known good unit as tested in two other A1226s. Each time I test the A1226 in question, I swap out the HD, RAM and IO power board from one of the other A1226s. I restore macs for a living so have several machines at my disposal for diagnostics. The solution/cause may be obvious but escapes me. I'm thinking the suspect A1226 has a faulty MOBO but what's wrong when an external indicates a healthy graphics system. Any ideas would be greatly appreciated.
Rob