1994 Olds Bravada won't start

94 olds Bravada, 4.3liter vortec engine, 130,000 miles was running fine. Started right up one morning and I shut it off when I got to my destination. Went to start it back up and it wouldn't. It would crank but wouldn't turn over. Had it towed to mechanic where he replaced the fuel filter, fuel pump, spyder, spark plugs, distributor, fuel intake lines, cut exhaust by catalytic converter cause he thought it might be clogged, it was fine. Welded it back together. Drove it for a day and the weld came apart. Took it back to mechanic where he re-welded it back together. On my way home it started sputting and spurting, made it to my driveway where it died. Called mechanic and we thought maybe something got clogged where it was welded so I cut it apart, then it started but wouldn't stay running. About 2 hours later it started so I drove it to my mechanic but it died halfway there. He is at his wits end cause he said when it is in his garage it starts but as soon as it is out in the cold it won't. Some of the days that it wouldn't start when it was outside it wasn't below freezing. We are going to try dry gas but what else could it be?Help please!