Sputtering 99 Venture

Here's my situation. I will go out first thing in the morning to start my van. The van will run for a few minutes and then shut off. I'll start it again and it'll run for a few minutes and shut off again. I will repeat this process until the van is warmed up. Now their have been a few times where I will start the van first thing in the morning and go driving and while accelerating the van will sputter. But once again, once the van is warmed up it runs fine. I took it to dependable mechanic and he couldn't fine anything wrong with it. The "check engine" light does not come on. I replaced the fuel filter and the PVC valve and that didn't cure the problem. Any suggestions.

Dirty fuel injectors?

I put in some super concentrated fuel injector cleaner and it seemed to work for awhile. Then the other day my wife was driving and she said it started to sputter and backfire until it finally stalled. I was taking the van to one of those express service centers and on the way up the van was acting really bad. When I would accelerate it would hesitate bad, then it would run for acouple of minutes then it would start to sputter and backfire then stall. I would wait acouple of minutes, start it up and it would run fine for awhile, then the same symptons would occur. Finally got it to the express service center where they conducted a fuel injection service and as soon as they were finished and started the van, it would back fire and sputter, lets just it was slow getting it home. The mechanic at the express center said it sounded like a catalytic converter problem. Let me add one more thing, my "service engine soon light" never came on during this whole episode. Does this sound like a catalytic converter problem? Would I get a code if it was? If not, could you or someone point me in a direction. My repair manual doesn't even mention "catalytic converter"