have you tried hooking up an external display? that can narrow down the possibilities, display or logic board. I've never heard of a bad HDD causing display issues, but have had broken, pinched cables, bad logic board. bad screen, but not a bad HDD causing it. try the external display & tell me the results
I hope you have luck with the G4FanControl. I used it just fine on a 1.67ghz Alum. PowerBook G4, won't work on any DVI, or ANY Titanium model I know of. I've seen other complaints about Fabrizi's "G4FanControl" not working on Titanium models. For that matter, her official site has complaints, just read ALL the testimonials.
Are you sure it's the trackpad? Have you ruled out the ribbon cable? Sometimes just re-routing it from under the hard drive enclosure casing to up and over the enclosure casing can correct trackpad problems. Have you tried that &/or replacing the cable with a known good one? A bad pram battery can cause erratic trackpad action as well as an overheating logic board. Is your laptop fan(s) working ok? You can download iStat Pro to check your working temperature. There is NO software to control the DVI's fan speeds, but you can check it's temp.
I'm using a 667mhz dvi PB with an 867mhz board installed. Works fine, even put in a superdrive from a 1ghz model. I'm not even sure which display I'm using. As long as they're the PowerBook G4 Titanium DVI 667mhz, 800mhz. 867mhz, or 1000mhz, REPEAT! DVI! models! Not the 667mhz VGA or the 1ghz Aluminum models, they're pretty much interchangeable. Some have a bluetooth module (1ghz), others don't. The screen resolution changes some between them, also. So, generally speaking, the parts are interchangeable, just not ALL. Do your homework.