Transmission not shifting properly after sitting and idling?

Great information here! Thanks so much for your response. It's happened four times over a two-year span. The first time was about ten minutes at idle before it happened. The others were just a few minutes. It's strange because it might go six months without it happening at all. I will definitely check the fluid levels and make sure it's good. Can I access this fluid check cap from up above, or do I need to be underneath the car to access? I mentioned before that I changed out the front motor mount, and rear transmission mount. Is there any way I could have gotten something misaligned by putting it back together? It didn't cure the vibration, so I was wondering if this could be related to this random shifting issue. I more than appreciate the diagram, and the other info you have given. It's a great thing you're doing here. Thanks for the help.

Thanks Joe. I had a chance to check out the cooler lines last night, and one or maybe both lines are leaking. They aren't large leaks, but probably enough to cause a problem. I also located the vent cap on top of the engine. At this point I am fairly sure that you have me heading in the right direction. I ordered new cooler lines last night, and some fluid to top it off after replacement. I will find the filler nut this weekend and also check levels. Do you think it would be safe to add a small amount before the check since we know there is a leak? Didn't want to do more damage running through the gears low on fluid. On the diagram you sent showing the filler nut, I just wanted to clarify that is behind the driver's side wheel. I have a good quality floor jack, ramps and stands. I was thinking one jack stand on the driver side should be enough to get it done, backed up with the floor jack. I can't say enough about how much help you have been. You said it, and bam, there it was! Leaking coolant lines! I would have never figured that out on my own. Thanks so much Joe! You deserve your expert status brother. I will let you know how it turns out if it doesn't rain all weekend. Lol.

Thanks for all the help, guys. I wanted to post an update for others that may have this issue. You were spot on with the diagnosis of transmission cooler issue. I found leaking transmission lines that lead into the cooler. I replaced those a few days back. (Got genuine GM hose assembly off Amazon for 20$.) I was able to check fluid levels as well. It was located on the passenger side almost dead in line with the passenger side wheel, about 6" to 8" up above tranny pan. If you go in head first, with the oil pan drain bolt staring you in the face, reach over and up to the left and there's a lone bolt on the side of the transmission towards the top. It would be easier to get to with the wheel and guard removed. But I was actually able to reach it with the car on the ground. I positioned the car over top of a dip in my yard and was able to reach it. Visibility wasn't great, but good enough to get decent reading on the levels. Ended up taking near a quart of fluid to get it to where fluid was barely leaking out in a small stream. Vibration has stopped for the most part. I think I need to make a small adjustment on the motor mount, and I think that might do it. Have let it idle several times while running in the store and back. So far, so good. It seems very apparent now that the vibration was coming from the tranny, and not the mounts that were replaced. I'm hopeful that the issue is resolved, and wanted to share for others. I do have one last question for anyone that may have experience with this. I lowered the engine all the way down into the front lower mount. For some reason, it seemed to feel a little smoother when the engine was riding a little higher in the mount. Not a huge issue. Just wondering if anyone may know the placement for that particular mount. Has a square slotted center in the mount where the bolt goes through. High up, or bottomed out is my question? Thank so much. You've been more than helpful. A true blessing!