When was the last time you changed your oil and spark plugs?
new plugs and plug wires really helped my 220 000km 92 DX,
What does the coolant level look like? is it low? Have you ever over heated the motor? does it eat coolant, this could indicate a blown head gasket and this will make your car run really Sh*ty
Could be an O2 sensor, does the car burn oil? BE HONEST, if it does you may need an new 02 sensor. but that would only bandaid the problem, as the oil contaminates the sensor to put it shortly. You could get some carb cleaner arosol can and spray that in to the intake track right near/after the air filter (don't spray the air filter you may ruin the filter see: paper and corosive liquids) as you hold the trottle (it will want to stall) this might clean the the throttle blades and MAF etc, and possibly allow the engine to burn off residues on other sensors, (like the 02).