1998 Buick Lesabre Very recent problem arose after using Fu

Hi,

I recently added STP Super Concentrated Fuel Injection Cleaner to my gas tank. The cleaner worked - the variable RPMs at idle ceased and became much smoother. However, at the same time (about 30 minutes of driving), the engine began to vibrate, which I was able to feel through the steering wheel. Also, the engine light began to blink, indicating a misfiring or something else. I have just about used up the gas that included the injection cleaner, so the injection cleaner will soon be completely out of the gas system. So far, the vibrating engine, which occurs more strongly when the car in in drive or reverse (and is less obvious when in park or neutral), has not relaxed at all. I am worried that my use of the fuel injection cleaner, while fixing one problem, started another. Should I be concerned? What is the problem and what is causing it?

Thanks for your help