Having a problem with my 98 GT

Update on the situation. I have cleaned the grounds under the hood, changed the fuel filter. No codes from the CEL. It has become steadily worse. The car will now actually die, instead of just acting like it. It doesn't matter if I am at a stop or going down the highway. It still starts back up again, but this has become a bigger problem. What else can I try to fix this? Could the battery be the culprit? Fuel pump? All help is appreciated.
 
Are the terminals on the battery tight? How old is the battery?

Did you pull plug wires to check for cracks?

Did you have the alternator tested? Do you have another key? IDK why the theft light would be flashing at the same time the car is started. WMBurns might know why the theft light is flashing. I would think the transponder in the key might be having an issue too.