'95 Engine Trouble

We have had this care for 2 years now, and after several replacing of spark plugs, the car is having trouble again. It is the 4 cylinder version. When started, the engine shakes a lot, misfires, and occasionally popping sounds are heard, especially when you take off after just starting it. When accelerating or coasting it runs better, but when you get to a speed and try to stay constant at that speed, the engine shakes and loses and gains power. If you accelerate, the more the better, or let off the gas the shaking and "lurching" stops. Is it bad sensors, a bad EGR valve, the sparkplugs again, or something completely else. Any help would be much appreciated.

Unless you are driving 30,000 miles a year your plugs should last 2-3 years or so. If your engine uses oil perhaps you have a seal leaking and fouling a plug because that's what is sounds like to me. However, it could be other problems, but unless you are seeing check engine lights coming on (even once) most likely your sensors are OK. Have you checked for stored codes? An oxygen sensor could cause this, map sensor, egr valve, possibly others but most of the time these will set a code to at least get your attention. Do your plugs look dark when changing them, or maybe one or two? A 4 cylinder car will act up a lot more with a weak or dead cylinder, so spark plugs are first thought if there are no codes set. Most car parts places can check this for you free although on a 95 I am not sure if it is OBDII or I. Most won't fool with it if it is OBDI as they don't keep older testing stuff onhand. You can buy a cheap tester for 20-25 bucks if so, but seems like 95 was when OBDII was being used. Check with Advance or Autozone to see. Maybe just a O2 sensor, may be the plugs again. have you checked for things like vacuum lines, air filter, dirty IAC (Idle air control valve) and have you ever replaced the plug wires? Might spray some water mist out of a spray bottle at night and let the engine idle, to check for bad wires that would have sparks jumping around the engine area.