Toyota Repair: 91 Toyota Tercel, toyota tercel, valve seals

Question
I have a 91 Tercel that is in great shape but has recently started running a little rough.  It almost seems like it is missing.  I changed plugs, rotor, wires, and cap but it has not improved.  The only other problem with the car is the typical leaking valve seals of the Tercel.  The timing belt was replaced a couple of years ago.  Could this be related to timing or a valve problem?

Answer
Possibly a valve problem, I would have the engine cylinder compression checked to see if there may be a compression loss to either a leaking valve or possibly worn piston rings, if the compression is ok then the leaking valve seals may cause excessive oil deposits on the spark plugs and cause a misfire, these engines are known to have piston ring problems so concentrate on that possibility.
Onw way to check is to remove the spark plugs and see if any of them have heavy black oil deposits on them.