1998 Hyundai Accent Electrical Issue

My car, a 1998 Hyundai Accent, has had what I call electrical "hiccups".
The former owners of the vehicle had to replace the battery and alternator, and I'm sure this was a symptom of what I have experienced as well. Whenever I drive or when the car is idling, it will have these "hiccups", some of which have caused the "check engine" light to come on.
A month ago, I went to have my "check engine" light checked, and the message on the reader said something about high voltage. At the place where I had the reading, they tested the alternator, starter and battery. Alternator and starter were fine, but the battery was below standard performance.
I had the battery replaced, and I found out I needed a new negative battery terminal. They put in a cap for my negative battery port for the terminal to fit in.
Recently, I had the terminal replaced, but it's still having the same "hiccup" problem.
I need help with that issue, but I am certain it is something with the electrical systems.