2001 random missfire

I have a 2001 with a 3.5 engine and about 140,000 miles.
about couple of weeks ago the SES light came on and the engine started missing, I ran codes and got a cylinder 4 miss fire and also a code for lean condition. After checking spark plugs and doing a lot of reading on here I decided to change out #4 fuel injector.
Now after changing the injector the engine still has a miss but it is different then before not as rough and I am getting new codes that list Random miss fire and High ignition voltage.
I went through and checked for vacuum leaks, removed and reseated the injectors (just in case I hadn't got them in right), replaced the spark plugs (it had bosch switched to NGK) and boots, I did also notice that the back coil pack had been replaced.

Any suggestions on what to try next?

Possibly the connector to the coilpack, and maybe the ground to the coilpack connector, but that might require a manual to determine where those 4-6 wires on that connector go.

I know on my scan tool I can read what's labeled as ignition voltage, but I believe this is just the "running voltage" of the car. 13-14.5V. If your getting a code of high voltage I think your probably over 15-16 volts. This could be something more to do with the charging system, or the battery gound that is right behind the battery. Follow that small black wire from the negative post of the battery.

I've never heard of that many coil failures on the 3.5L. I wonder for some reason if your car's running a high voltage, causing coils to blow.

I'm sure others on here will have some comments as well. New coil packs are really expensive. It might be worth a call to a parts store to see if they can test them. I don''t know if you can check the 3.5 coil, using a standard multimeter or not.

ok to update this one,
I replaced the front coil pack with a used one and also the spark plug boots, that helped but was still getting the random missfire code, and also now when going over 30mph the SES light start flashing, I can put the car into neutral, let the engine rpm's go back to idle and the light flashes until I drop below 30.
I stopped by Oreillys to use there tester and they have a new "policy" where they charge you $250.00 to take there tester out of the store and I'm in an area that auto zone isn't in so I don't have easy access to a code reader. ANY SUGGESTIONs on what to try next?

You really need the code to get at the problem, otherwise you are simply guessing. I bought a code reader from harbor Freight a few years ago when my Intrigue was acting up. It's basic and can only read and erase the code but with forums like this, that is all I really need. It cost me $35.

Or you can try E-Bay: http://cgi.ebay.com/ebaymotors/OBD-II-SCANNER-OBD2-2-SCAN-TOOL-CODE-READER-OBDII-ENHAN_W0QQitemZ380205194160QQcmdZViewItemQQptZMoto rs_Automotive_Tools?hash=item5885fedbb0