I have green lines across the length of the tv from top to bottom when using the av channels these become more prominent in the darker hours or when the room is dark, they are seen more easily when the picture is dark.

Id start by replacing the tcon board, its the small board generally at the top of the set. sometimes under a metal cover

sounds like your panel is going bad. usually not economical to repair.

The coolant in between the CRT face and the focus lens has probably become discolored and would need to be drained, cleaned and refilled. The green CRT is the center one and it would have to be romoved in order to refill it.

You have a CRT TV
CRT's get dim with AGE. This is normal.
Your TV is getting old.
NO fix. Get a new TV

This problem just resolved itself after TV being left on for an hour ????? Probably will need looking into anyway.

do the lines go from top to bottom, and are they about 1 inch apart? or do they move diagonally across the screen?

convergence board is failing

No. You have bad convergence output ic;s and probably at least 1 burnt resistor. This liely will have to be reconvergec in the servcie menu after the aprts are changed.

I’m happy to help further over the phone at https://www.6ya.com/expert/dewayne_4a10eab6caecdb92

Do a reset to this TV, and then check. There are two methods to reset TVs. The first is simple to do; a Hard Reset. The other is somewhat skilled; is a Factory Reset (System Reset). To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight is better. This procedure will make it a hard reset. If you want to make it a system reset [Factory Reset]; you have to enter its service mode option, and select the factory reset option. If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/

Your problem could be a bad LCD screen, and beleive me will get worse with age. It could be a Driver IC is not putting out the proper drive for that matrix of TFT. If you have a warranty, stand by your guns as its not right, so call and have the servicer make it right. Good Luck PS please do not forget to rate this thread.