Blowing Maxi Fuses

My 97 3.0 Vulcan decided to quit in the middle of a short trip for no apparent reason. Checked fuses and found that a 20 amp underhood maxifuse in the Power Distribution Center had blown. These are not labeled and my Haynes does not seem to identify each fuse. It was in the back row, last one on toward the driver's side of the car. I replaced it with a 30 amp, but that blew after about 5 miles of driving. Put in a 20 amp, and that blew after about 3 miles. Obviously, at $2 a fuse, this is not much of a solution.
I suspect this particular fuse has something to do with fuel injection or fuel pump. The car cranks fine, but will not start until the fuse is replaced. It does not seem to get gas.
Coincidentally, I have been getting a "purge control valve" error code [0443, if I recall correctly] for more than a year. I just replaced the valve this weekend, but it did not remove the code or affect the fuse situation. I can't tell if they are related in any way.
Any clues are appreciated.

I know this is not a answer, but from the way the fuse is blowing over time. I would say somthing on that fuse is getting hung up causing a large draw. If it was a short then the fuse would blow as soon as it got power. Another one I have seen acting just like this is the rad. fan. The fan bushings\bearings go bad and then the fan gets a few turns in then hangs up and pop's the fuse.
Sorry not a ford guy, just looking ta help.