I keep killing A4988's please help! (2 replies)

Hi all,
I have a home built 3d printer. It uses Arduino mega, ramps 1.4, and A4988 step controllers.

things have been going along nicely until last weekend.

I was doing a print that took about 24 hours, and when the print was finished, I homed the print head.

x and y worked great, but when the z started down, it was very erratic, and then it just stopped.

I ended up swapping out A4988's and then it worked fine for about 10 minutes, then it too died.

I swapped it out again, and this time I made sure that the pot was turned down to limit the current.

I have printed two pretty big projects since then, but this morning the Z was again dead.

I swapped out my last good a4988 and made sure the pot was turned down. I also put an amp meter inline.

my 2 Z motors were drawing 120mA. I homed the Z and it moved about 2 inches and then died.

does anyone have any ideas as to what I'm doing wrong?

I know that prior to the first controller dying I had a Z crash. ( homed outside of the bed too far) but it ran for a few days after that.

any ideas on what I have hurt?

thanks.