My car keeps turning off?

i have a 2000 lesabre that keep turning off on me. i drive great when its on but for the last 4 day it shut off while im drivin. the power stays on and after i put it in park it turn back on like nothings wrong with it. this morning it happen and after it turn back on its try to die again but then ildle real high then went back to normal. no check engine light is on i gave it a tune up about a month and a half ago i took it to a couple of shop but cant seem to get it fix. yesterday i did have a va replace because it was broken we tought that was it but it started again this morning, does anybody know whats wrong with it, i need help asap