Various circumstances. Sometimes from a cold start will run 30-60 seconds, then stall. May stall again after immediate restart. Sometimes at regular speeds or at higher speeds on highway I will see tach needle quickly jerk all the way down trying to stall, then abruptly recover, then jerk down again etc. before either stalling or being O.K. for the rest of the drive. Sometimes after a stall it won't start again unless left alone for a couple of hours. Occasionally goes for several days with no stalling at all. I've kept up with general tuneup stuff, swapped out computers, air mass meters, and fuel pump relay switch, checked sensors, ran vehicle with sensors unplugged. This has been going on for about a year now.