I've got the same symptems.
Not solved it yet. I've replaced ignition leads, spark plugs and fuel filter.
The spark leads tend to last about 10 years for originals, check to see if they look old. Also check the distributor cap for cracks.
The fuel filter is a 3year service part I think so you could see how old it looks. Its under the car behind a plastic shield near the back.
The other thing to check is the fuel pump or fuel pump relay, it seems the fuel pumps can cause intermittent problems.
If the problem is worse with a nearly empty gas tank its probably a fuel supply problem.