Well with the mileage on this ride & the unknown maintenance/repair history, all kinds of things come to mind, from plugs, wires, coilpack breakdown, to worn timing belt, to O2 sensors & cat converter clogging, to sticking or burned valves & we could guess all day long & not do much good, so imo you may want to consider beginning with some meaningful diagnostic testing.

Hook up a scantool that'll read live PID's & have a look at long & short term fuel trim, engine temp sensor reading, MAF sensor output, O2 sensor switching range & speed & scan the computer for any set, or pending trouble codes. Maybe have the scantool do a cylinder balance test while your at it.

If you don't have, or can't borrow a scantool, hook up a vacuum gauge to a handy port & see what the vacuum gauge has to say.
If you've never used one, here is a link to a site with some moving gauge graphics & how to interpret what the gauge is displaying.
http://www.secondchancegararge.com/public/186.cfm

By then you should have some idea which system to move to the top of your suspect list.

Post your results here & we'll all offer up our next best guess on how to proceed.

More thoughts for pondering.