It's not the plugs & it's probably not the coil.
Plugs won't act that way & the coil should trigger a CEL for a missfire.
The gas cap idea is good. Though very loose/leaking and it throws a CEL as well. (Evap Control leak/etc)
My guess is the IMRC box is failing. Once it's get's nice and hot it doesn't open or doesn't hold the secondaries open.
Since it's closed at start up it won't throw a CEL (P1518) & since it works occasionally it won't throw a CEL (P1519) either.
