I just got a 3rd lamp on Craigslist, and the lava/goo would warm up with the normal stages of spiking and slow flowing, but when it got fully hot it wouldn't rise from the bottom of the bottle/globe. All 3 are 32 oz. midnight series. #1 is cream lava with blue liquid, #2 is red lava with yellow liquid, #3 (the new one) is yellow lava with purple liquid.
All 3 have very similar size and shape metal springs/coils at the bottom. I'm not yet comfortable enough opening the bottle and replacing the coils so I decided to try something easier first - changing the bulb
#1 never had trouble flowing right.
#2 would sometimes get too hot and separate from the coil at the bottom and a large blob of lava would just sit in the middle of the bottle and do nothing. I figured it had something to do with the light bulb (heat source) and I read that 40 watt frosted bulbs would not heat as much as clear bulbs, so I changed the bulb from clear to frosted and it rarely if ever over heats anymore. When it does over heat, I remove the bottle and let it cool down for a couple minutes and fall down to unite with the coil. Then I place it on a smooth flat surface and gently spin the bottle - no pun intended - to make sure the lava is completely attached to the coil. It usually works fine after that.
#3 wasn't working right from the beginning, with a 40 watt frosted bulb. I could see that the lava didn't have an even consistency after a long time - there were still small and large chunks of lava that were not thoroughly melted. I also noticed that when a bubble of lava did rise off the bottom, it would very quickly fall back down. Overall the lava seemed very bottom-heavy and I thought "this lava is not hot enough!"
I noticed in some of the other discussions that there are many different bulbs and techniques to get the lava flowing right - try different things and go with whatever works. So I bought a 40 watt clear bulb and a 40 watt spot light bulb. I put the spot bulb in first and the lamp seems to be working fine now. If it starts to over heat, I will put the clear bulb in and give it a go.