Let me start by saying this...I don't drive my XJR that much, but when I do, it is just me in the car. No passengers. I had swapped out my seats last summer with VDP seats, front and rear. The fronts worked fine...well, I take that back. The drivers seat had some phantom issues that never went away, but the passengers side slid forward and backwards just fine. A friend of mine, last summer, sat in the passengers seat after I had swapped out my wood trim. Some change fell out of his pocket and onto the seat. I picked that up. Nobody sat in the passengers seat since then, UNTIL a couple of weeks ago. Had to take my other car somewhere, so drove the XJR so my wife had a way back home. She tried moving the seat and it went forward a bit and then the left side of the track kept sliding forward, but the right side was "STUCK"...wouldn't budge. Then after a second or two, the left side was stopped moving. Slid the seat back, and the left side moved back and then finally the right side slid back.

Thinking something was stuck in the track, I removed took the bolts out of the front of the seat today and tilted the seat back. Didn't see anything stuck in either side. So I slid the seat forward just a tad to access the rear bolts....took those out. Checked both sides and didn't see anything at all stuck in the tracks. Played around with moving the seat forward and back with either the front two bolts in, or the rear two bolts in....NEVER with all 4 bolts out at the same time. Same issues. The two sides seem to not be lining up because the right side just stops. When the seat is all the way in the "back" position, both rails stop in the same spot and are even. Once you start sliding them forward, they will move evenly and then the right side stops and the left side keeps going.

NOW, with that said....if the seat is in the full back position, and you slide the seat forward while pushing the right side from the rear, both sides will slide all the way up. To get it to slide all the way back, you have to either push on the dash with your hand or push your foot to the floor in order to push the seat back while using the power button. It then slides back all the way.

When doing this, you can feel a roughness in the ride side of track. It will be smooth then all of a sudden feel like it is "catching" something and be "bumpy"/vibrate. Then it will be smooth again, then it will be "bumpy" again...it feels like this when you go forward or backwards.

Ended up getting tired of looking at a ghost, nothing that I could see stuck in the track, etc. Not sure if anyone has any suggestions or?? Any help is appreciated.

PS - This has nothing to do with the power control module under the seat.

 

I've seen this with replacement seats from parts sellers, the seat's lower sliding rails are fragile, seats get dropped and these get bent, either way it sounds like the frame is bent or there's something in the track binding.
Seat out and check the runners with a flashlight and also check if bent. Bumpy? could be a worm worm gear.

 

I was checking it out for over an hour yesterday. Didnt see anything in the track at all, looking from the top, bottom and sides. Didnt notice anything bent either.....but could the rails become "uneven"? For example if all 4 bolts are out and the seat was slide forward or backward, would one rail move to a different position than the other one? Or would they both move the same?

 

Your symptoms sound like a drive gear with some broken cogs. If you have the x308 tech manual there is a procedure for disassembly and inspection. I have included a link to my drop box where you can download the X308 Tech Manual if you don't have it.

https://www.dropbox.com/s/9lq7amaw90...kshop.pdf?dl=0

 

I will have to download and take a look when I have a few spare minutes. Where is the drive gear that you are talking about? Is it that black rectangular unit that is attached to the right side seat rail?