First, it would be nice if you told us what the last cable was that you didn't connect.
Re the black resdiue, if it was from burning, there is virtually always the characteritic odor that goes with it.
I know nothing about RDRAM, but I would think that the speed of that RAM determines the front side bus speed. I don't know if it gets two reads per clock or not, so I'm not sure about the FSB issue.
Did the system ever run at 800 MHz FSB? Should it with your memory??? If it is 400 to 800 that would mean that the 400 or 800 would be determined by what speed memory that you have.
I would think the running at only 100 MHz is a default safety mechanism, built in, such that if a bad shutdown is detected the system defaults to a very safe setting. [although I really think that it would be 200, unless that is the memory bus and it is double clocked like DDR and 100 memory bus is called 200 at the FSB]
Describe a PSU tester that somehow catches a bad floppy connector, wild! If you are only running at 100 MHz FSB, the CPU is likely running slower and thus consuming considerably less power. Possibly your Power Supply can't hack the full power configuration anymore. They do degrade, and 300 Watts doesn't give a lot of margin.
Really need to know what the last cable's function is.
My prob started with my PC not booting. All I would get would be the COMPAQ logo. At first I thought it might be a HD failure because of bad coincidences (like leaving a subwofer under my PC), but I connected both my HDs to another computer and they are fine. Then I thought it might be a PSU problem. But I was told if that were the case I wouldn't even be able to see the logo, it just wouldn't turn on. So I bought a PSU tester and I guess they were right. It showed no problems other than a bad floppy type connector that hooked up to a firewire that I don't use. This last part was today...
So what I tried, again don't know if it was a coincidence or not, was to not hook up this last cable. I turned on my power and my PC is up and running again. I don't think the prob is over tho because there are signs inside the case, as well as outside the case where the fan sits, that something overheated. There is black dust residue, like when wires or components burn up. I haven't tried to do anything heavy yet figuring that once I rev it up it'll go down again. Also, I noticed that my system info now shows a system bus of 100, where it used to show 400 (tho it should show 800, but that's another matter).
So that said, anyone got any ideas on what else I might look for?
P4 1.4GHz (400-800) BUS