I have shower/tub combo. One knob pulls out to turn water on. Then you turn knob left to make water hotter and right to make water colder. I adjust the temp of the water into the tub easy enough. Then when I divert the water to the shower it comes out extremely hot. If I turn slowly to the right, it stays extremely hot. At some point though it turns to extremely cold. Mostly though it is extremely hot. Yet when I divert back to tub, I can control the tempurature easily.

I would start by replacing the cartridge. Is the diverter on the tub spout?