'99 Express sluggish

My work van with 5.0 V8 with 76k miles is acting sick. Occasionally, but not all the time when accelerating it hardly wants to go. The nearest thing I can think of to describe it is it acts like an old motor with the vacuum advance not working. If I go on and put my foot in it it will finally take off and go. Just bought a 20' trailer to pull with it and I want it running better.

I first thought it was tranny so I had it flushed. No help. I have run plenty of SeaFoam and B12 and Heet through it. I have changed the fuel filter. Occasionally I am getting the engine light on and it gives a MAF code. I can clear the code and it will not light up for a while with no difference in running habits.

How many miles ? Better or worse in wet or dry weather? Are you due for a tune-up?

76,000 miles. Has been doing this while it has been dry the last 2 months. Wish it would rain, we do need it. I could see how it does then. As you know, it is a van, so I've never taken off the doghouse and done any plugs or wires on it. It will do it with the engine cold or hot.

I'd do plugs, wires, cap, rotor, PCV valve, and air filter after that many miles and that many years. Consider also the fuel filter as long as you're getting dirty. Time to take off the engine cover and do the work.
FWIW
YMMV

Thanks for the ideas, looks like I might as well get dirty or broke.