Firstly, when you use a UTH, remember that your main concern is the temp at ground level, where the snake will be. Unless you provide a lot of climbing equipment and the Corn stays up there for most of the time, then you don't have to worry about temps more than an inch or two above the UTH.
Secondly - where is the probe for the thermostat? If you have it on the tank floor immediately above the UTH, then you can set it to the required temp and it should hold the warm side steady there.
Don't worry about the cool side dropping during the night. If your Corn is too cold, he'll move to the warm side. That's the point of having a heat gradient - the Corn can choose for itself. As long as the warm side isn't dangerously warm, he'll be fine.
If you don't have a stat - get one, because that's how you hold temps steady. 90 degrees is a bit on the hot side. You could use the stat to keep it a little lower.
With a UTH and a stat, you shouldn't need the lamp as well. There's not much point heating the higher parts of the tank if the Corn won't be using them. And unless the lamp is also controlled by a stat, that will be an unregulated heat source, which could contribute to overheating.
