hello all, new here.
i've been having some issues with my car lately so heres the story

When i start the car and proceed to put it in reverse the cars rpms race before it hits into gear, and it does this in drive also. BUT not all the time, so one day i drove around did it for a while then it threw a code, got it checked it was a 502 code ( vehicle speed sensor ) so i finally changed it out, the old one looks crappy so figured this may do it, drove it around didnt do it then it did it again, got the code cleared figured maybe the car needs to relearn it, didnt help. so i checked the fluid im pretty sure it needs to be changed because its a brownish color, so i added fluid ( was low ) and put some non slip in it thinking it could be internally... but if it was internals of the trans wouldnt it do this all the time? it seems to do better when the car is warmer. when i drive everything else is fine, only when i come to a stop does it do it going to drive, and when i drive there is no slipping im at a loss right now bc my girl has it in Connecticut and im on long island... so i cant help her

any help would be great guys. thanks

Year, make, model, mileage, and history on the car (prior to your ownership) would provide some more useful background info.

It'* a 96 buick regal with a 3.1 v6 it'* got like 124k on the clock.

I don't have any previous info other than my gf said it overheated one day due to a radiator issue and the intake manifold gaskets were just done the issue with shifting was before intake man was done.

could be a dirty IAC try cleaning it or replacing.

But if it was an iac issue wouldn't the rpms jump around at start up? The rpms only race when trying to drive (foot on gas in gear) then it engages

Its not an IAC. Do you have any idea how this transmission has been maintained? I would start with a fluid/filter change to see if it improves the condition, but you may need a rebuild.

That'* next on list when I get my hands back on it when she is here. Any other ideas because the issue got a lot better when I changed the vehicle speed sensor

A slipping transmission is sign of an internal mechanical failure, or low transmission fluid. Sometimes you can improve it with an oil/filter change (throw in some Lucas). The first step is to make sure the fluid is up to the proper level. If that is not the problem, change the oil/filter. If that doesn't fix it you may be looking at a rebuild.

Read this write up: Part 1 - How Can I Tell If My Automatic Transmission Needs an Overhaul? (GM 3.8L)

But if it was internal wouldn't it do it all the time, to be doing it intermittent and only when cold for most part to me sounds like a sensor issue, I'm going to change the fluid and filter maybe the fluid so old it'* guming up shift solenoids perhaps?