the obp setting "auto-boot?" means, if true, the mashine boots automatically after power-on or reset. so that cannot be the X issue. anyhow, a solaris installation does not set the auto-boot? variable to other values (OBP isn't touched by solaris unless you change anything) ....
it could be that you lost your OBP settings for any reason, so that OBP is set back to defaults. then you possible miss your "output-device" setting in OBP to your gc..
2nd - what graphiccard are you using? perhabs the fresh installation is missing the drivers for your card
gP