'91 spluttering like an old bi-plane on idle after moderately warm
 

I have a Ford Ranger in decent shape, low milage but yeah still old.
Lately, when getting somewhat warm say after 2-5 mins of driving, when I come to a stop sign or just let off the gas, it splutters like an old prop plane and if I don't keep a foot on the accelerator, it will die and won't start up till after I let it set for about 5 mins. Or maybe I just get more determined to make it start then and crank it longer?
I replaced the air filter which was fairly dirty. That seemed to make it work better for a bit. That might have been illusory though.
It also does the spluttering thing when thoroughly warm if I'm going down a hill with foot off the gas.
It runs great otherwise.

I would check your fuel pressure.Should be 30-45 psi.

is that easy to do? I've never done it.

You can either borrow a gauge from a parts store or buy one. On the fuel rail, there should be a shrader valve, it should have a cap on it.You then screw on the gauge's hose onto the shrader valve. Do it on a cold engine since there could be some residual pressure remaining in the fuel system as some fuel can squirt out. Cycle the key on/off a couple times and start the engine. The pressure specs are at idle.Afterward, bleed the pressure out with the relief button on the gauge before disconnecting. I advise you to squirt the fuel into a coffee can as you relieve the pressure to prevent a fire if gas ends up on the exhaust.