Hi Keman, welcome to the forum.
The next time the fire burns out and won't keep going, clean the firebox out and check out what is on top of the grate. Are the gaps and slots in the grate clogged with ash?? This is what happens with my grates, after a week or so, if I don't very aggresively shake the grate, it will eventually clog with hard crusty ash. Soon the air is blocked and the fire goes out.
Since you changed the grates then have had this problem. Check to make sure the grates are hooked up right and are getting full movement with the shaker handle. I'd watch the grates with no coal on them and see what motions with the handle give the most effective grate movement.
If you can see the bottom of the grates from below through the ash pan door, you might be able to keep the grates open with a stiff wire poker. Use the poker to dislodge the ash in the dark, cold areas in the grate, the hot, red areas are getting enough air.
Hope this makes sense and helps. Greg L