Can the effects and symptoms of traumatic brain injury come and go, on different days?

Sort of...: Usually the defecits are stable, they don't change. However, things like memory and attention can be effected by stress or mood states to when you are calm, the defecits may seem better but on a stressful and anxious day, they may seem much worse.

To some degree: Most of it in seer injuries should remain pretty stable bit even normal people vary in terms of mental skills and medications, sleep, stress and intervening variable such as depression or anxiety can weigh in. Severity makes a difference. People are bad judges of their memory - neuropsychological testing often tells a different story. In legal cases, there may be another explanation.