Hello azharj,
I'm glad you found the information somehow useful.
I think your first problem is the installation of the check valve, it should have been installed the same way the older one was. It is designed to provide vaccuum from the reservoir to the firewall (the dashboard) and to keep it from going back to the engine. If you installed the valve in a different position you must still be loosing some vacuum. I would install it horizontally again.
The other problem I think might be related to the MAF sensor if it happens only when hot; you can try disconnecting the harness going into the MAF and running the engine like that (it may turn off the first times, just start it again); it is located after the air filter inside the big plastic tube and has a rounded connector attached close to the battery (that's the connector you need to disconnect). If you find out your windstar runs better with it disconnected when hot then the next step would be to give it a good clean with some electric contact cleaner, MAF cleaner (if available) or some other non-oily and none residue cleaner. Failure to use only these types of cleaner will damage the sensor (if it's not already).
Regards,
Oscar.