Acceleration issue

I drive a 1999 Saturn SL1 (automatic). It drove fine for about a year after I bought it, although the radiator did need to be replaced when it suddenly blew. We had that done, and it continued to drive fine.

After awhile, it would "slip" or "double-tap" when I accelerated quickly, such as when entering the highway. We took it to a shop for a diagnostic, and the mechanic put some anti-slip fluid in the tank (free of charge). The slipping stopped for several months.

Then one day, I stopped at a stop sign, and when I went to proceed, I realized I was barely accelerating even though the gas pedal was floored at this point. I could hear the engine revving, and the RPM gauge went nearly into the red. I quickly stopped and turned the car off, then restarted it, and the problem stopped, for a bit. On the way home, it started to do the same thing, but corrected itself.

I've had oil changes done regularly, and the fluids all seem fine. The check engine light is not on, however, it was on not long after the radiator replacement, and we cleared it because the mechanics told us that it was just "stuck" on. I cannot remember the exact numbers, but I remember that they were listed as "Incorrect Gear Ratio" ( for second and third gears).

I've only driven it a couple more times, and the issue has only happened once more. I was entering the highway and it simply would not accelerate or shift, even after stopping, shutting down, and restarting. Not wanting to exacerbate the issue, I got it home, parked it, and have not driven it since. It always starts up fine.

What could be causing this, and how to I go about testing possibilities?