It could be the AFM. Out of curiosity, I unplugged mine and tried starting my 4Runner to see what would happen. It starts up and runs for a few seconds, then dies as you have described. I don't know how to test it, but if you get the wiring diagram for it you should be able to figure it out. You could have a bad connection, or broken wire between the AFM and the computer too.

Here's another idea. I got water in my computer and my 4Runner would start and run really bad for a short period of time then die. Each time I tried starting it it ran for a shorter and shorter period of time until finally it wouldn;t start at all unless I let it sit for a while. I dried the computer out and it worked fine.

Have you checked your codes?