Truck Repair: Blowing the ignition fuse on a 8600 international

Question
Have a 8600 international, an it keeps blowing the ignition fuse. We have checked for wires that have rubbed though, an can't find anything! The truck will run if it's sitting, but if you drive it, it will blow the fuse, an if you put a new fuse in it will blow it, let it sit for a bit an you can put a new fuse in an it's fine, than take it for a drive it does the same thing!  Any ideas?

Answer
Just throwing out some ideas. Any corrosion on any connections causing it to draw too many amps. What about a bad starter or starter solenoid could be shorted. The one thing i have dealt with is a bad wire altogether. I've had them corrode inside the casing and break inside the casing which causes a short. Or i've had the wires over time get weak and the wire gets hot causing problems with fuses. If there's anything aftermarket patched into the ignition wire unhook those to make sure nothing aftermarket is causing it. Alot of times to eliminate the wire i'll make a quick jumper wire to bypass the original wire to eliminate that as a possibility.