When car is idle. Seems like its going to die out.

New muffler was put on a few weeks ago - but not welded on. Clamped. It rattles like mad. Was just wondering if that has a connection to my problem. Just this morning when driving to work - car idle at long stop light. Seems like car was going to die out. Was leaving it in neutral and drive, neutral and drive - so it would die out. Finally got going so car never died out.
(Overall seems like in good condition, but when a/c is on and at idle position sounds like it will die out also and sounds like it trying to cycle or something. But I just turn off a/c when that happens. A/c belt changed already.) Too. But thats not the problem I was initially worried about.
Don't want car to die out when idle at stop light in future. Can muffler (or parts near it/catalic converter or whatver) be the problem. Or is it an engine problem?