It could be anything related to the ignition system: coil, plugs,
wires. If the plugs and wires are original, now would be a good time
to replace them. It could be the coil also. The coil pack consists of
3 seperate coils which is shared by 2 cylinders, cyl 1&5, 2&6, and
3&4. So while your problem is not caused by a common coil (cyl2 and
cyl5 do not use the same coil), it is still possible the coil is
defective. You can try testing it with an ohmeter using this test
procedure.
http://www.autozone.com/servlet/UiBroker?ForwardPage=az/cds/en_us/0900823d/80/0d
/c1/7e/0900823d800dc17e.jsp

Just an FYI, when you get to the part where you check resistance
between each coil, and it says the norm is between
6,500-11,500 ohms, a normal reading will  actually be about 13,200
ohms. This was discussed many times in this forum from people who
tested a known good coil.