Recently I bought a 2000 Ford Ranger with the 2.5l engine and automatic tranny with 150,000 miles. It has a rough idle at time when in park and runs rough at lower speeds especially at about 30-40 mph. If I give it a substantial amount of fuel it is almost like it is delayed because it runs rough for a sec then smooths out as I go faster. Any ideas as to what might be my problem before I just start throwing parts on it.

 

Lots of unknowns, so Don't begin throwing parts at the problem, take the time to perform a proper trouble shoot, find the culprit Then replace it.
Have the computers scanned for pending, or set trouble code clues & post All code Numbers, as those code clues can shorten up a trouble shoot & lower repair costs.
Most autoparts stores will scan the engine computer at no cost if the CEL is lit, some will do it if we're having problems without a lit CEL.
Seeing as how this ride has 150K miles on it, its Way past due for a good bit of scheduled maintenance replacement parts, like air & fuel flters, PCV valve, O2 sensors, spark plugs & wires, ect, ect, so where did the previous owner have it on scheduled maintenance & where are you right now on getting all that taken care of???
Our waste spark ignition system is persnickety about the spark plug design it gets, so best to stay with OEM plugs & wires, as they're of good quality & designed for the Double work load the waste spark system puts on them. Best also to stay with OEM air & fuel filters. BTW, does it now have an oiled foam, or cotton gauze type air filter?
Some thoughts for consideration to get your trouble shoot started, let us know how it goes.