Welcome to the forum.
Let's start with the basics - the bucket load of cash may not happen.
When the warning light is on, the transmission is in 'limp home' mode, or to put it another way, in 'safe' mode. When in this condition, it will keep to safe parameters (ie, it will not change properly, and will probably only shift up to 3rd) because the
has detected a fault somewhere in the system.
However, because the warning light does go out, the fault may not be permanent (positive sign).
In my opinion, the first thing you should do is change the transmission fluid.
Once that's done, see what happens and keep us posted.