I'm hoping someone here might be able to give me some options to check on this one. This thing has me stumped. Up until a few weeks ago the car has started AOK every time for the 80k miles we've owned it. Not that it may make a difference, but the last big repair we had done (clutch and engine seal) was just a week before this started to occur. This has been going on for about 3 weeks. It's NA, 5 speed.

Symptoms:
After sitting 4-5 hours the engine will not start promptly. It will turn over and try to start but shudder for a 1-2 seconds before stopping. Turn the key again and it fires up immediately - like it should. Starts AOK for a couple hours after that. Obviously this becomes an issue commuting to work and home.

Things I've checked so far:

1. Fuel pressure at rail for ignition-on (pre-start) is 40 psi immediately, during running it's 46 psi. Turning off the engine it drops immediately to 40 psi. It takes about 30 minutes to drop off 6-8 lbs.

2. I installed a new fuel filter - just in case.

3. Battery connections are tight and clean.

4. The Fuel Pressure Regulator does NOT have and return fitting. It has only one fitting that goes to the vacuum tree. The replacement FPRs DO have a vacuum and return fuel line fitting. I find that odd since it just sits there hooked to vacuum. The vacuum end seems to be intact - no gasoline in the line.

5. No other obvious symptoms such as codes/lights. Mileage is good, acceleration/power is great through all RPMs. Idles smoothly. No gasoline or unusual exhaust smell. Ambient temp has been about the same as before.

Being that this started so suddenly has me wondering what changed. It will start just fine for 1,2,3 hours. It's the long wait when it malfunctions.
Any suggestions that I might be able to check would be appreciated.
Many thanks in advance!
JF