The coil rail is hard to diagnose without just swapping it out. If the coil rail is shorting internally, it affects 2 companion cylinders. It still runs fairly smooth but it lacks power.
When they first start to fail, sometimes it only lacks power under heavy acceleration, but it soon lacks power at all loads. But there are a couple of ways to test it without swapping it out.
1. Have someone with good hearing stand next to it, hood open, while you rev it - snap it. If they hear a snapping - spark jumping somewhere, replace the rail - it's shorting internally.
That still doesn't mean it's good if they don't hear anything.
2. Next, use a laser thermometer (harbor freight $29), run it at 2000 or so for 3 or 4 minutes, then go underneath with the thermometer.
Measure the temp of the pre cats. They should both be about the same temp - around 300 degrees. If the coil rail has been shorting it's been dumping raw fuel into the cat - the cat will run very hot - sometimes in excess of 700 degrees. The other cat will probably be cooler than normal, 200-250 degrees. If yes, replace coil rail.
And then there's always just trying a new one.
If you have to buy one, pay that little extra for a good one. NAPA, CarQuest sell rails made by Standard Ignition - decent quality - same mfgr the factory uses.
But there are cheap knock-offs - Chinese trash - Autozone and the like sell them.
You'll save around $30 over a real one.
If you count your savings you'll really save money on them, every time you buy one you'll save $30 - in a years time you'll save at least $90! They don't last very long.
If you buy quality and pay that extra $30, they'll last years, it will only cost that extra once. (I wish the government understood that principle.)