The purpose of this study is to determine whether certain experimental treatments can help treat autonomic dysfunction symptoms (symptoms relating to nerve damage of the autonomic nervous system) associated with Long COVID, also known as post-acute sequelae of SARS-CoV-2 Infection (PASC). Long COVID is defined as symptoms associated with COVID that have lasted for at least 3 months and occur or worsen at least 1 month after infection. The study focuses on adult participants with Long COVID who experience postural orthostatic tachycardia syndrome (POTS), which is due to autonomic nervous system dysfunction. POTS causes a number of symptoms, such as a fast heart rate, dizziness, and fatigue. Participants will be randomly assigned to receive either an active study drug or placebo (inactive substance). IVIG (a form of intravenous immunoglobulin, also known as Gamunex-C) will be evaluated in this study; it is approved by the U.S. Food and Drug Administration (FDA) for various conditions, but its use in patients with Long COVID is experimental. In addition to being assigned to the active study drug group or the control group (placebo), participants will have an equal chance of being randomly assigned to the coordinated non-drug care group or the usual non-drug care group. The coordinated non-drug care will involve lifestyle changes, such as diet modification, physical activity, assisted care and weekly phone calls with a care coordinator. The usual non-drug care is the usual (general) healthcare guidelines that participants may receive as part of their routine healthcare.