I've been looking for anyone with a similar issue by haven't seen it.
This 90 Ram 5.9 tbi will start after it sets for a day or several days,it runs beautifully, then all of sudden it just loses spark and dies. sometimes it happens when you turn on another accessory like the A/C, sometimes it'll just dies while idling, and sometimes it dies when driving. Then it wont restart. At first I thought it was acting like it lost fuel so I tried priming the tbi with fuel with no effect, so I'm sure it's losing spark somehow. Where's the best place to begin looking to try & diagnose this issue?

 

my dads truck use to die whenever it would rain but his would alway fire up again.
have you checked the sparkplugs? have you checked the fuel filter this could cause it to stall while idling or while driving.

 

When it won't restart check the spark at the plugs. Also look inside the throttle body and check the injector spray pattern. An intermittent injector problem can cause what you describe. If one of these is wrong it will at least give you the proper direction to go in.

 

I don't believe it's a fuel problem because I've tried a couple shots of starting fluid and it still doesn't fire. I haven't had a chance to start diagnosing yet since it's hanging around 115° the last few days.

 

You asked "Where's the best place to begin looking to try & diagnose this issue? ". I gave you the very basic things that must be present for the engine to run. If you rule out looking at the very basics first good luck.

 

Check your Hall's effect coil-- the pick-up coil in the distributor. Symptoms of a defective unit are: Out of the blue it just quits, anytime any place. Pull over and leave it maybe five minutes or five hours and it fires right up again. Goes along smoothly again for five minutes, five hours, or five weeks........really has a mind of its own. When it quits get in there immediately and check for spark at the coil. No spark? Pretty good guess it is that little devil. Had the problem first with a 1979 Volare (5 pin ECU)and had a spare distributor ready. Ran it in the driveway and luckily within ten minutes it quit. Had the opportunity to plug in the spare distributor on the spot and diagnose the problem. A few years later I acqured a 1989 B150 with TBI and it started the same games.........just changed that coil and the problem disappeared for good. That was about 12 years back.

 

Thanks Seal, I was only pointing out that I think its a spark issue, not fuel. I know I have to start looking for lost spark 1st at plugs, then coil, dist, etc. I just thought I could get some input from someone who maybe recognised this type of strange behavior, something like lilred experienced. I have 2 extra trucks the same year & motors setting in wait for me to restore, so now I can experiment with a couple simple parts & see if it helps before we have to spend money.
I have two different manuals for this truck & one says the dist must be removed to change the pick-up coil and the other does not say, is it necessary to remove the dist to do this?

I also hoped to get some kind of ideas here so when it cools off here in Az a little I'll have a direction to begin following.

 

I can't say for sure about the 5.9 but I have changed the pick-up coil in my 318 while still installed in the truck.

 

Same distributor. P.U. coil can be changed on truck, although it is easier with it out. Especially easy on a van.

Check for "No Spark" at the coil first, then work back, it will eliminate a lot of searching.