1998 Chevy Astro 40 amp fuse blowing.

I have a 1998 astro with 138000 on it. I run a daily route of 188 miles. I can run my whole route and it will start every time although it seems to hit hard when it starts. Kind of like a punch instead of it turning over as normal. Now if I run the first 50 miles and turn it on oand off a time or two for stops not a problem. Then run 15 miles still not a problem other than it seems to start that punch starting. Again another 15 miles and the same thing. But the next 30 miles when I get to this stop and turn it off deliver the stop anywher from 5 to 10 minutes same as all the others it blows the 40 amp fuse under the hood. I replaced the starter recomended by the mechanic that started me by crossing the starter off. But it still does it. Have you ever heard of anything like this before. Also it seems like it might do it more with the heater on. Also replaced starter relay. It seems like its some kind of combination between distance drivin and how many times the ignition is turned on. If you just sit there and start it without driving it you can start it over and over and cant make it blow. Blew 3 times in front of same store only once at another store. Ive been told aything from ignition switch, to the ecm, to the crankshaft position sensor, to something to do with the heater. Just dont want to start putting a lot of high dollar parts on that are not neede. Just hoped you might have come across a similar sitiuation. Thanks.