I was going to post an issue like this, but decided to bump the old thread instead.
I just had one like this. Same model and all.
The unit heated and then cooled and didn't heat again.
This unit uses an odd valve/solenoid combo. See at first I didn't catch that, and went to take a solenoid off and got that wonderful smell of gas . So yeah, first time with this issue on this solenoid type.
I figured it was the operating limiter on the Flame tunnel. These buggers will have flaky bi-metal that works and is in continutiy at room temperature but open quicly and take longer than normal to reset.
Since for LG and Samsung these limiters are so cheap, I went ahead and swapped it.
The dryer heated up and ran just fine, for one load.
Realized the issue was the coils. I have seen this same issue in Whirlpool dryers caused by coils. Hell we all probably change coils once a week.
Yet these coils were different. Had to cut gas off in the back, a lot of unscrewing pipes and such.
After Iswapped it out, there were no issues.
So if you have an LG gas dryer, with the issue of unsustainable heat after initial heat-up I would look into the coils.