I saw some awesome conversation on this site that is very similar to my dilemma. Unfortunately, I have not found the root cause of my problem. Trying to wire in a GFCI outlet in bathroom. 6 wires. Identified the line and load circuits. When trip C/B in sub panel, I also lose power to 2 bedrooms, the switch in bathroom for lights, and the overhead fluorescent light in kitchen. Wall in bath is shared with kitchen. Hook up GFCI with just the line wires and GFCI works as it should. Hook up the load wires and nothing, outlet trips when hitting RESET. After finding this site with StingyB's similar problem, I did some investigation but don't have any GFCI's in those other rooms. Do I maybe have an outlet wired wrong in one of the other rooms?
Noting how you speak about Line and Load, it's clear that you have identified some of the wires as Line wires. Ok so far. However it appears that you are identifying all other wires as Load wires. that is not true. Granted, many GFCI instruction sheets mislead you in this area.
Actually all the wires are LINE wires, and all of them should be attached to the LINE terminals. The warning tape should be left on the LOAD terminals.
The only time you should attach to the Load terminals is when have identified a specific load that you actually want protected by GFCI, and where you have reasons for wanting to do that.
You could have an outlet wired wrong downstream from the GFCI. It would likely be a neutral-ground reversal, which a standard tester will not detect.
Other options are a fault in an appliance or fixed wiring - check everything for damage, and try it with everything unplugged.
You could also have a neutral going to another circuit, which is sometimes common with three way switching.
Consider splitting the circuit apart further down, to narrow down where the fault is.