1994 Cougar no start

'94 Cougar 3.8 is dead in my driveway again. Has spark, and fuel pressure. Cranks fine, but will not fire at all. This has been happening intermittantly for the past month. Car will run fine for a week or so, then will not start after sitting 8 hrs, or overnight. Then 2-3 days later, car will fire up first try like nothing is wrong. Today is day #3 that it's been dead, this time. Have spent many hours researching online stuff & forums, but have found nothing that helps. Have checked EEC-IV codes, but today only get long term codes 173/177. Checked all electrical connectors under hood. Pulled one injector plug & it is getting voltage when cranking. Cap/rotor/plugs are only 2 yrs old. Fuel pump is new. Starting fluid did nothing last time I tried it, so did not try this time. Any ideas? This is my daughters car, and she is not pleased that Dad can't fix it.