Intermittent starting problem

[I4Veritas]

I have a 1990 Legend. It sometimes won't start and this happens frequently enough to be a problem.

What happens:
I'll turn the key and the car will begin to start. You can hear the poping of ignition and then it suddenly dies. However, the starter is turning over. If I let it sit for 20 minutes (sometimes up to a couple of hours), it will start up fine. This seems to happen randomly. It has occured in the morning and afternoon; when it's humid, and when its dry. It happens once or twice a week.

Other problems (may or may not be related):
The check engine light has come on every once in a while. I looked at the ECU light under the passanger seat and it blinked 6 times, which is the coolant temperature sensor. The check engine light does not stay on, however. It is very occasional.

After first starting the car and then driving below 25mph, I hear a light clickty clickty sound that speeds up as the car accelerates. It goes away after a couple of minutes.

What I have done:
Had it tuned up (didn't have a starting problem for a couple of weeks).
I have had the dealership look at it and a local mechanic, but they are never able to duplicate the problem (of course).

Does anyone have any ideas as to which components in the ignition system might cause this behavior? I have heard the crankshaft posiiton sensors may be a culprit, but it takes several hours of labor to get to them (I have 3) and they each cost more than a hundred to replace. Any ideas are greatly appreciated.

[ND1980LS]

It's the Main Relay without a doubt !! It's happened to both my Coupes and is very problematic with the Legends.