Got this from http://autorepair.about.com/od/troub...gine_drivn.htm
Symptom: The engine surges or misfires while moving.
The engine seems to start fine and will normally accelerate fine. As you are driving and maintaining a steady speed, the engine seems to "speed up" slightly or it seems to miss and buck.
1. If you have a carburetor (there are still a few out there), the choke may not
be set properly, or the choke may not be working correctly.
The fix: Check the choke plate and make sure it is opening completely.
2. The engine may be running too hot.
The fix: Check and repair cooling system.
3. The fuel pressure regulator may be operating at low pressure.
The fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure
regulator. (Generally not a DIY job)
4. The ignition timing may be set wrong.
The fix: Adjust ignition timing.
5. Ignition system problem.
The fix: Check and replace distributor cap, rotor, ignition wires and spark plugs.
6. There may be a fault in the computerized engine control system: Check
engine control systems with a scan tool. Test circuits and repair or replace
components as required. (Generally not a DIY job)
7. The fuel filter may be partially clogged.
The fix: Replace the fuel filter.
8. There may be a vacuum leak.
The fix: Check and replace vacuum lines as required.
9. Possible internal engine problems.
The fix: Check compression to determine engine condition.
10. EGR valve may be stuck open.
The fix: Replace EGR valve.
11. The fuel injectors may be dirty.
The fix: Clean or replace fuel injectors.
Your problem can happen to anyone of us. Hope this can help.