The motor runs awesome at all speeds including WOT but die occasionally at idle speeds for no reason. At that point the engine will fire back up for 1 second and then die again. It doesnt matter how many times you try and restart it the engine will run and then die...If I put the throttle all the way open and try and start it the engine wont fire (just turns over) but dropping the throttle all the way back will get the engine to start but only for a second. If you let it sit for 5 minutes it will fire back up and run perfectly. No codes either and I had my injectors flow tested last summer and all are good. Plugs, wires, cap and rotor are all new.
What does this sound like? IAC valve? TPS? Vacuum leak? Its just not consistent--I can run it at idle speeds for 10 minutes and have nothing go wrong. What does this sound like?

Sounds like vapor locking. Does it do it when its hotter outside or after it has been run for a while and then shut off for a short period of time???
You probably need the Merc booster pump.

That or do you have the VST system???? If so do a search for VST. I think the screens plug up and cause this problem.

Sounds like vapor lock to me also.

In 94 merc came out with a kit to help with some of the vapor locking, But it did not really solve it completely.

One of the reasons the engine would not try to start while you had the throttle all the way open is because if the ECM sees a certain percentage of throttle while just under 400 RPM's in crank mode then that will put the ECM into clear flood mode by turning off the fuel injectors.

Most Merc/Volvo EFI engines are set up to do this at different percentages of throttle, Some are at 75% and others are at 100%

Now had you said I could open the throttle up to say around 5ish % and the engine would start and run, Then It would sound more like you had a bad (IAC).

I live in Seattle (colder climate) and its NEVER happened (yet) up here. All of these issues were occuring at Desert Storm in Havasu while cruising the channel. It was about 90-95 degrees out...
I asked this same question to a local shop and he said the same thing--vapor lock. He suggested I remove the checkball from the fitting which attaches to the tank. Im going to try that first...Not sure if I have a VST. What is the Merc booster pump you mention above? Fuel pump?