I know this is a Ford Forum but, the navigator is the same animal. I have a 2003 lincoln navigator and after driving it for a while, when moderately accellerating about 40mph the transmission seems to slip out of gear the enginge will rev up and not engage the transmission. You can either wait and it will eventually shift into gear or manually put it into 2 gear and it will engage. If I wait for it to stop I believe it is becasue the vehicle conditions warrant a change to lower gear and that is why it goes away by itself. I think it may be the TCC solenoid but would like some confirmation of that. This only happens after the car has been driven awhile and the transmission is "warm" and the same speed everytime about 40mph ( the overdrive shift I think)

 

If it were your TCC the vehicle would stall when you come to a stop. Try checking your trans fluid level. Make sure the vehicle is HOT and in park, check the fluid level at idle. If its low, you might wanna dd some more

 

if the TCC solenoid was "on" I could understand that it would stall at a stop, I was thinking that is sometimes not working or not engaging overdrive gearing. I have already chekced the fluid level it was the first thing I checked. It is definitly one of the shift solenoids or TCC solenoid cause it keeps happeing at around 40mph around were a shift into OD is supposed to happen during moderate acceleration.