I have an automatic1989 S13 Silvia and about 2 months ago, i was driving in congested traffic and my car all of a sudden started making this whining noise.

It got higher pitched as the RPM went up, but it only made the noise in 1st gear. I slowed down because of a red light and went to creep closer to the car behind me and noticed my car was going slower than usual.
When the light turned green, i went to accelerate but my car didn't drive, it just made the whining noise and sat there as if it was in neutral. Luckily a guy helped me push it off of the road and into a nearby servo.
I got the car's transmission serviced but they couldn't find any problems with it, and my dad (mechanic for over 20 years) couldn't find any fault codes in the car. I recently got it back and it drove great, but yesterday i was driving it and it seemed to make that same noise again but not as loud, and sometimes the whine sounded jumpy.


I have no idea what is wrong, can anyone help me out? Thanks!