I30 infinity 2000 transmission goes into neutral then stuck in 1st gear

The transmission is probably gone, when is the last time that the transmission fluid was changed?

Have the same problem in my 2000 I30 Infiniti I noticed it was doing the same thing you're saying I believe it might be the range sensor need a transmission I just ordered it waiting for it to come in to the mail going to install it and I'll get back to you to see if that's the problem also it might be the wrong transmission filter in your car

Also it could be your speed sensor I believe it pays off just a little then replace the transmission if it's not the transmission speed sensor in the range sensor it's probably less than fifty bucks each I would try that first I'm in the process of doing that already change the speed sensor did not fix the problem already change the fuel in the filter and still didn't fix the problem now changing the range sensor hopefully that will be the problem so wish me luck

My I 30 runs good for about 20min or until it warms up then it will start hanging in 2nd gear so I was wondering if that range sensor helped you out any?