Help! The weirdest thing started happening today...

[freewave]

I have a 2007 Yaris. Base model, auto, 2 door, only owner. The last three years not a lot of maintenance has been done. Oil changes and fluids only, sorta thing. Has a small exhaust leak, but it hasn't been there long.

This morning I started her up, and it sounded normal. Then it stalled when I brought the gearshift down past neutral into drive. I started it again, and again everything sounded fine. I feathered the gas as I brought the gearshift down and it went into drive. Drove to work normal. When I got to work I tested it to see if I could replicate it, and I could.

I searched on here all day at work, but couldn't find anything really relevant. A coworker suggested it could be the spark plugs, so I changed those myself as well as the air filter. Both were overdue.

When I started it back up, the engine sounded fairly "off." Like it wasn't firing right. It immediately stalled when shifting into drive.

What could cause this?

Thanks in advance!