Recent ATF drain/refill, now car shifts extremely weird.
 

Recently had a ATF drain/refill at my local Infiniti dealership @ Massapequa, NY on my automatic 2003 G35 sedan.
After a month of driving I seem to have extreme slipping issues with the transmission.
Whenever I would press hard on the gas the car would rev extremely high up to 5k and the whole car would shake and it would seem like something is grinding. Driving on Manual Mode would not produce this problem. Upon checking the ATF fluid, I noticed it's a dark brown color (upon searching, people said that drain/refills would make the fluid that color) and looking at the dipstick, I would notice that the fluid would reach the first curved mark when warmed up.
https://cimg1.ibsrv.net/gimg/g35driv...f677a2030c.jpg


I recently bought an OEM tranny dipstick for my 2003 as they did not originally come with it. Could it be an ATF overfill that's causing the slipping issue or is my AT shot?

Would like to ask here first before bringing it back over there and have them charge me just for looking at it.

Normally slippage is a symptom of low fluid but I never had shaking like you mentioned.

that's what I thought too until I checked the levels. I am going to try and record it happening to see if anyone has an idea of what it is.

Last time I looked AT fluid was red, it only gets brown/dark and fugly when you've drive your car 30+ thousand miles! This is only a guess, if you've only driven your G a few thousand miles they never serviced your AT and I'd take it back!
Gary

Upon doing more searches I have found out the issue is called
Transmission shudder.
The speed would near 40mph then you would feel the transmission shake the whole car and it would slip.
When searching I found several remedies which include
MAF cleaning (recently had a CEL light due to a dirty sensor, had it cleaned and did a idle air relearn, CEL lights off afterwards. could it mean damage even if CEL lights are off?)
Repeat drain/fills
The controversial atf flush
ECU update

does anyone else have any suggestions on what to check for?

Up to you bud. Sounds like fluid caused the problem, so maybe it's due for replacement.

If the transmission was overfilled it is possible for some foaming to occur. How many miles were on the car when you did the flush?

Shudder is a sign of clutches worn to the point of needing a rebuild. Usually an AT will shudder for a week or so and then just completely give up a gear. Would be interesting for the OP to post what the problem was since it happened 2 years ago.