91 B2600I auto transmission question

I have a 1991 B2600i with automatic transmission. When I put it in drive or come to a complete stop. It will sometimes act like it is in neutral and then jump into gear. Fluid level is full. Does not smell all that great and looks dirty. Fluid was changed about 2 years ago. Any ideas? Is it just time for a rebuild? Any trouble shooting that I can do?

Change the fluid and filter, duh?

Any concerns with mileage of near 300I with changing fluid?

Is that 300K? Is this your truck and this just started? How original color is the fluid on the stick, pink, brown, near black? If just dirty looking, hope it is a clogged transmission filter. I do not know a lot about transmission workings. Does it run and shift fine other than this problem? If so, I probably would invest in filter/fluid change.
May be the modulator, but I lean toward the filter first, That many miles and not knowing when it was changed last, that would be my first choice.
I have recently bought one, towed home, had set 10 years (since Katrina). Had no compression on cylinder 3, others good. Did a valve job, intake valve on 3 was corroded open, problem fixed.
After I started it, the transmission fluid was WHITE (condensate, doubtful, flood water, probably). I change the fluid five times, each gallon jug I let transmission pump out the transmission cooler line got pinker, but the reason I tell you this, in the bottom of the gallon jug every time, there was a half inch of black "stuff". This came from the torque convert, sludge. Mine stopped shifting, so I have the engine on Craigslist, to put a 4.3 v6 into it. If it does not sell, I do have another transmission.
So yes change at least the filter, even if, to save the cost of fluid (I used 5 gallons) you think the old fluid is good enough to reuse, ONLY TO CHECK THE CLOGGED FILTER IDEA. If this fixes the problem, changed the fluid, wash out the new filter and feel lucky.
May well be the transmission is going. Keep us up to date.

300,000 miles. Tranny fluid was changed in march 2015. First gear is slipping and then hesitates to go to 2nd. Different things at different times. I appreciate the feedback. I am looking to find someone in Houston, tx to rebuild. I might just sell it. Not sure.

Sound like it is time for that, but then the engine may be as tired. Perfect time for a v6 upgrade [cost of transmission rebuild, engine rebuild, more than buying a v6 with transmission from salvage yard, or buy a running v6 truck and use the engine/tran (do not use van engine, a/c and alternator are too high)]?
I say v6, because that is a no modification to the truck swap. I have done 4.3 automatic swap. The only change I had to make was the cross member which goes under the oil pan. Hood closes, used same radiator. i have measured and it appears a standard shifter will be in the right place for the console (all these truck have the mounting shaft under the dash for the clutch pedal), I just went with the automatic, because it is easier on the rearend. A shift kit in an automatic will twist the tires enough to please .
On GM, power steering pump is right there by the sector for hose makeup, A/c is on other side of engine. I do not know about Ford.

So I had the transmission rebuilt. Ending up costing me $1500. It runs fine but shifts hard between second and third. Taking it back for some adjustments

OK. Have had truck for a bit now. It has gone back to the shop 3 times. It shifts and runs great at highway speed, however, there is hesitation on take off. I mean it hesitates to shift and then after it shifts, it will do a quick jerk. Kinda of like a shift kit hard shift. The shop is telling me that they have gone through three valve bodies and still the problem is there. He asked me to change ignition switch, which I did and the problem is still there. Anyone have this issue after a rebuild?

I've got the exact same problem with my B2600 G6 from 2003. It stopped for a while but it comes and goes. From what I've read it's pretty common with these trucks and I haven't seen anyone post online anywhere with the fix. Except maybe getting a new transmission at the yard.