your right, they can't be replaced. If you're only throwing p0300 code (multiple/random misfire) it's really hard to tell where the problem is. hopefully when you pull your plugs,it will tell a clearer story

 

Its a coil pack going bad. I had the same issue,once the car warmed up the coil would short out and run ruff. Then it just ran bad all the time. The flashing SES light means you are potentially doing damage to your cats. With the miss fire that cylinder is dumping raw fuel into the exhaust and then cat has to burn it causing extra heat in the cat.
The steady light means you have thrown a code. My code indicated the cylinder that was missing but if yours is just the general miss fire code you can buy one coil and start swapping out the coilpacks till you discover the bad one.

Here is how to reset and read the code if you dont have a reader at home. I would try and reset/clear the code and see if it comes back identifying a cylinder. i.e. 0301 would be a miss in cylinder 1.

http://www.technosquareinc.com/350reset.htm

Good luck.

 

^see i would have said that but like you said, its a random misfire code so it makes me think that it might be a vaccum leak. I would check alllllll your hoses for cracks, wear etc. before dropping $180-$240 on coil packs to find out it aint the problem.