My 1996 Subaru Legacy wagon has an unstable gas gauge - but only when it’s hot out. All summer, the needle will slowly drift back and forth between deep below “E” and half or quarter tank, so that the only way I can tell how much gas I really have is to watch the trip odometer. It doesn’t do this in the winter. What could be the cause?
I speculate that you have a bad wire somewhere. Or you need a new gas gauge.
The cheap way is to continue watching the odometer. Cost: Free.
Or have a mechanic do a full diagnostic. Cost: $$$$$, but with peace of mind included.