My dads 98 12 valve, Fuel Indicator light continues to come on, when tank is 3/4 full or below, gauge goes to empty, then then goes to full, has anyone had this problem or know what is goin on?

 

The fuel sender in the tank needs to be replaced, common problem. Costs about $40 from the dealer but they will tell you that you have to replace entire $300 module.
Don't listen to them.
Biggest hassle is that you'll have to drop the tank or tilt the bed up (prefered0 to get at it.

 

The same thing happened to my 97. Was in the process of replacing fuel gauge in tank when I pushed on the connection on top of tank and it started working and has been ever since. Don't know if this will work or not for you but might be worth a try prior to replacing fuel gauge.