My '97 5.9 idles fine in park or neutral, but at a traffic light (and at operating temperature) it idles too slow and stumbles every 30 seconds or so. It also seems to crank longer than it ever did before before. Any ideas?

First, check your fuel pressure, then, disconnect the battery, take the throttle body off, and thoroughly clean it. Put it all back together, and see if it doesn't run any better.

Fixed your plenum yet?

I'd suggest checking the crank sensor. Behind the passengers head between the block and the bell-housing. 2-8mm bolts I think. If it gets debris on it it'll fire late...stumble for a second. Then two...then start tentatively.

I'm not saying don't do the plenum as stated above, but peer down the TB and see if there's any moisture under it in the intake.

I took the throttle body off and completely cleaned it, along with the 3 removal attachments (I don't know what they're all called). I put it all back together and it immediately idled at around 1700rpm, then slowly (about 4 minutes) came down to around 1000rpm. I took it for a 20 minute drive and some full throttle accelerations and I think it's much better, but still not 100% perfect. I'll know better in the next couple days though. I am the original owner, and have never done the plenum. There is a light oil film in the intake, but definitely no pooling. It's looks the same as when I first looked down the intake, about 200,000 miles ago.
I'll check the crank position sensor if I determine it's still not 100%.

I don't know how to check my fuel pressure...

Oh, 97..... I think you have the plastic rail, that didn't have the test port.... Makes checking the pressure truly an exercise.......

^ I concur. Checking the pressure on a '96 and up is a PITA ^

The 97 seems to be the red-headed step-child of the 2nd gens..... it is the only year for the plastic fuel rail, with no provision for the pressure gauge. My 96 has one.

I've got a spare '95 hanging in the rafters of my shed!

My bad. I was thinking 96 1/2 (Model Year and ^)

Yeah 98+ uses the metal rail with the test port.