staninoly

My '95 Legacy 2.2 occasionally won't start after it's been driven. Engine cranks ok, just doesn't fire. Let it sit for up to 45 minutes, starts right up just like nothing was ever wrong. It'll do this maybe once in 10 or 15 trips, hot or cold weather - same. No CEL. Coolant temp sensor, right? Replaced it. Same problem. Next, following leads from my Subie guy and posts on numerous forums, swapped in another ECM, then replaced the knock sensor, then swapped cam position sensor with my '97Outback hoping to transfer the problem to it. Nope, still the Legacy. I've checked spark and fuel delivery when it's doing it's no-starting thing and both OK. Seems to be a common but elusive problem. Any ideas?