My son's '98 Ranger, 3.0L, standard tranny blows the starter relay/anti theft fuse on occasion. When this happens he replaces the fuse and when he tries to start it the fuse blows as soon as the starter is engaged.

This has happened 3 or 4 times, he found that if he is shoved and pops the clutch to start it, it starts normally for up to six months sometimes 2 or 3 before out of the blue it blows the fuse again.

He's had it to mechanics but they can't tell anything because it acts normally when they have it.

Any Ideas?
Thanks