Tranny Problem?

I was stopped at a light today in my 87 4.0 automatic. When the light changed I stepped on the gas and heard a whirring / ginding sound from under the truck and it wouldn't move, forward or reverse. I put it in park and got out to wave traffic around me. I got back in and went to put it in neutral so I could push it off to the side. As the selector passed by reverse I felt the tranny engage. I put it in drive and it drove away like nothing happened. About an hour later the same thing happened. And it just happened again. It feels fine when it's working, the fluid is at the right level and doesn't look burnt, and it doesn't slip at all (at least not when it's working right). In my 30+ years of driving this is only the second automatic I've owned, so my knowledge is limited about them. Any ideas?

I always start checking for low fluid (are you checking the fluid level with the motor running?), next a plugged filter screen (change the oil). Then I double check the shift linkage (cable) adjustments (there are stories about the shifter mount welds coming loose). The TV (transmission valve) cable adjustment, controls pedal to the floor passing gear and pump pressure in all gears. Also check the transfer linkage, I have found neutral, in other than the neutral position on the transfer shifter (often a sign of bad motor or tranny mounts).
Oddball things that have caused me trouble are a flatend oil cooler line, also once a plugged cooler line.
I try the easy stuff first.
If it´s a worn pump, it usually takes an FSM and some pressure gauges to troubleshoot.