I'll take a stab at it, villages! :-)
Screen dimming: It could be what you're suspecting - the converter/inverter itself. Could also be the cord connection at the laptop, either internally or actually in the cord's connector itself. Wiggle the connection/cord a bit when the dimming problem happens to see if that causes any change. Another potential: If the dimming happens constantly, it could also have to do with screen reaching the end of its useful life. Also, does it happen only when the laptop is running when plugged into the wall or does it also occur when it's running only on battery power.....or both? That could help narrow things down a bit.
This might be a lead on what could be causing the speed change:
http://www.overclock.net/intel-cpus/...ed-change.html
Maybe that'll help a little......maybe not!
Not an area of expertise for me - just offering some logical ideas and feedback from a little bit of research. Maybe someone with more specific knowledge will jump in as well.
Bill