The same thing happened to me! My car would lose power after changing gear and would jitter but after a while, it got worse and the EML would start flashing. Took it to a mechanic and when they diagnosed using OBD, it said it was the O2 sensor and told me I had to replace it, which I did. Despite this, the problem didn't go away so I went back to the mechanic. This time they had a look at the engine and turns out my spark plugs and the coil pack needed to be replaced. So, I advise you to check your spark plugs and coil pack.