[James-Z4Lover]

Hope one of the BMW Guru's on here can help me with this one...

2006 Z4 3.0Si, 65,000 miles, 2nd owner, very little issues, driven mostly only on weekends.

About a month ago, was on the highway, car was fully warmed up, started driving her a little hard, downshifted to 4th, accelerated and tried to shift up to 5th, she lagged made a slight "squeal" then finally shifted up to 5th and into 6th. Then slowed down a bit, tried again, and she stuck in 4th gear, wouldn't shift up or down, and the yellow transmission light came on.

Couldn't get her to shift at all, going into Neutral and back to D kept staying in 4th gear. So I put in Neutral, pulled off to the side, turned her off, waited about a minute, started back up, transmission light off, and she drove fine. So I drove her cautiously home w/o any further incidence.

Now, it periodically does this and seems to be getting more and more frequent. I believe it's only 4th+ gear problem, as I haven't noticed an issue w/ gears 1-3. Seems turning her off and on always resets the light and the issue, but then will happen again pretty quickly sometimes.

I've researched online and read about two potential causes:

1) Low/old transmission fluid, even though BMW says the ATF is "lifetime". Others have said this may cause shifting issues and recommend changing every 50,000 miles. I read that Valvoline Maxlife ATF is an excellent choice for the Z4 and plan to attempt that change this week. ANY TIPS here?

2) Low battery - read somewhere about potential Low Voltage may cause some electrical transmission issues. She has been starting a bit slower each time I start her over the past ~6 months but does turn over each time. New BMW battery about 2 years ago. Current Off voltage reads 12.1V, which seems low to me, thus the slow starts, but I wasn't sure if this would cause any transmission shifting issues, as I assume the alternator would provide adequate voltage was running - is that true? (I'm not at home w/ the car now, so can't test the running Voltage, but will next time I start the car).

Has anyone else experienced this and was able to solve it? With the hard shifting and "sticking" between gears, it feels like a mechanical issue to me, but being able to turn off/on the car and have it drive ok (at least for a short period of time) makes me wonder if this is an electrical problem, or a sensor, or ...?

Appreciate any help on this one, as I'm at a loss.

BTW, also ordered a OBD2 reader from Amazon recently, so can hopefully check codes soon. Although the Check Engine Light is NOT on, so not sure any codes will show up...??