2005 Tahoe- transmission problem
 

I have a 2005 tahoe z71 with 130k miles and recently had to replace my flex plate due to a crack. I noticed around this time my tranny fluid was dark brown with a red tint. I know that is not the color it should be but now my vechicle when Im at a stop and try to accelerate it seems my car doesn't move for a second and then the transmission kicks in. It does this maybe a few times a day. I have no issue with any other gears while the transmission shifts but want to make sure this doesn't get worse. What does this sound like? I'm having my transmission fluid changed along with filter this week and hoping that fixes it.