2004 Aveo bucking & jerking like I need to downshift

I have had my Aveo for about a year, bought used. 5-speed manual transmission. I don't know much about auto mechanics, but have passable basic skills. But I have learned the value of DIY, and I have really enjoyed working on my Minion.

So, for a couple of days, I have seen a ridiculous lunging, jerking, bucking feeling: and it's gotten worse. At first, I thought I was just tired from the holidays, and forgetting to downshift, or not smoothly letting off the accelerator. It has been happening when I let off the gas a bit. If I am accelerating, or the clutch is in, no problem. And today, it actually stalled when I took off from a stop. What the heck is going on?

I should include that I recently replaced a heater hose that sprung a leak. Maybe I knocked another hose loose somewhere? Timing belt is snug, teeth intact. Or could this be misfiring? Haven't had the sparkplugs changed since I bought it. Oil is a bit overdue to be changed too. Could it be that simple? Or is this something that's going to cost me a kidney?

Run that gas tank clean. Put a new tank into it from somewhere else not from where you bought gas last. I suspect it's crappy gas. Not to be confused with crappy grass for that bad trip.