4L60E transmission issue

[Dankyogi]

I am having two issues with my 2000 GMC Sonoma 2.2L 2WD single cab pickup truck. It has the 4L60E transmission in it, which is what is causing issues. It currently about 146,000 miles on the truck itself, but has a rebuilt transmission in it with about 60,000 miles. The reason I replaced the transmission the first time was because it wouldn't shift out of first gear and there was a loud cranking, grinding sound. Reverse also wouldn't work. I first noticed issues with the original transmission during a 400-mile road trip across generally hilly terrain. I wasn't towing anything, nor was it loaded with much except a large suitcase and my wife and I.

About midway through the trip (traveling 60-70 mph), what I believe was the TCC failed to engage. I'm a complete noob when it comes to transmissions and TCs, but research on the web led me to surmise the TCC was the issue. As I was in the fourth gear, the RPMs jumped up from 3k to 4k, with no change in the accelerator position. I was not at WOT. The tachometer would remain at the higher RPMs for about 30-60 minutes until it would randomly drop back down to the normal cruising rate. This cycle would repeat multiple times throughout the trip there and back. Not realizing there was a major issue (and being pretty broke), I didn't have my truck checked. About a month later, the transmission did the grinding, clanking sound and went caput. That's when my mechanic recommended a transmission replacement.

That was about six months ago.Two weeks ago, I took a job that requires me to travel about 20 minutes on a 70 mph interstate to get to work and back. I also drive the entire time throughout my 8-hour work day. The temperature outside has been nice (70-75 degrees), and I'm never revving the motor past 3k RPM.

So that's the history. Sorry for the lengthy explanation. I wanted all the facts in there.

Here's the two problems I'm encountering.

1: Hard shift into second gear.

The transmission jerks when it shifts into second gear, but it doesn't happen all the time. It seems to happen after I drive for a while, or travel at interstate speeds after cruising around town for a bit. The odd thing about this is that when I turn off the truck and let it sit for about five minutes, the problem disappears, and doesn't return for more than an hour or two. When it happens again, I just park, turn off for five minutes and continue on until the next time it begins shifting hard.

2: TCC disengaging again on interstate.

This has only happened once and it was today. But I don't want to destroy another transmission waiting to figure out the issue. I was traveling home, and at about halfway through the trip of 20 miles, the RPMs jumped from 3k to 4k at a cruising speed of 68 mph. Again, there was no change in the accelerator and I wasn't running WOT. But I had driven the truck for about six hours straight (minus a stop here and there for restroom, lunch, etc.). When I got off the interstate, the hard shift into second was even more pronounced. I then pulled into the nearest parking lot, let the truck sit off for five minutes, and the problem vanished.

Side note: The SES light came on about three weeks ago, stayed on for three days and went off. Earlier this week, the light came back on. I haven't checked the code yet. That's this weekend's task. I will update the post with the error code when I get it.

Soooo...

Anyone have a clue what's going on? I sure don't.