any ideas, blows fusable link

well i seem to have a intermittent problem on my 68 barracuda. when i got it the original fusable link was toast and bypassed. i put in a spade type 30 amp fuse. anyway it has a intermittent problem of blowing the 30 amp only when the key is in the run position only. doesn't do it on accessory. i've unhooked the alternator, voltage regulator, changed the ignition switch, i just pulled the cluster and it appears ok. the amp gauge still works. when it pops the fuse it goes all the way to discharge first. it does read normal when you turn the lights on, cigarette lighter etc. i had the car running the other night and the amp meter seemed to work normal. went to light the car back up and out the fuse went. just looking for ideas of what else to check. ive been through all the wiring under the hood, converted it to elec regulator, dual field alt. this was obviously a issue before i got the car though when everything was totally stock..