Sharps

I have an 01 Cherokee 4.0 six that I use as a DD. Last year I rebuilt the motor - completely including new O2 sensors, cam & crank sensors, injectors coil pack. I runs fine starts good so no real problem. I run about 70 on the highway as I go 25 mile one way to work. The problem is if I accelerate hard getting on the highway the CEL will flash but goes away. This morning it flashed then stayed continuous. Code reader gave 0300, 0304, 0305.
Where to start?

extrashaky

The '00 and '01 computer will often throw a tantrum when something goes wrong and throw multiple red herring codes when there's really only a problem in one cylinder. First thing I would do is clear the codes and then see if it remains a multiple misfire or settles on one cylinder. If it remains multiple misfires, it's more likely to be a fuel or spark issue that affects the whole motor. If it settles into one cylinder, you know where to focus your attention.

Then I would go through the normal tests (not necessarily in this order--switch it up depending on what you see as you go):

Pull and inspect the plugs to see if there's a fouling pattern that can tell anything.

Check the fuel pressure at the rail.

Test compression to see if there's a bad cylinder.

If you can get your hands on a known good coil rail, swap it in to see if the problem goes away. Do you have a buddy with a coil rail motor who would let you borrow his coils for a few hours?

If you narrow it to one cylinder, inspect the injector on that cylinder and swap it with a good cylinder to see if the misfire follows the injector.

Test and/or clean the throttle body sensors and replace as necessary.

Check the camshaft position sensor to make sure the magnet inside hasn't come apart or loose.

Get a toothpick and check the indexing of the camshaft position sensor. (Search "reindex camshaft position sensor" for more info.)

Remove the valve cover and inspect the valve train for broken springs or rocker arms.

There was a TSB for our years for weak springs and an issue with valves not rotating properly, resulting in carbon buildup that keeps valves open. You might search for that TSB and see if it applies. You can also try clearing carbon by using Cruiser's steam cleaning method that involves pouring water into the throttle body while the engine is running.

I'm sure I'm missing something, but that's everything I can think of right now. If you don't already have them, you can borrow test kits from Auto Zone or similar store for testing fuel pressure and compression.

Sharps

Thanks Extra, I cleared the codes last night - good so far this am drive in to work. I will look at the plugs tonight and proceed from there.

extrashaky

If you have a scanner that will show pending codes, don't wait for the CEL to light up again. Scan it after you drive it to see if there's a code pending.

I had a cylinder six misfire that caused the computer to throw a hissy fit and show all sorts of red herring codes. The first indication that it was actually a cylinder six problem was a pending cylinder six code before it threw any others. Eventually it settled into just cylinder six code every time I cleared it, but it took a few times to settle in.

Sharps

I changed the plugs over the weekend - hadn't been in very long and looked just fine all look the same so no indication of misfire. I will test the injector voltage next - What is the best procedure for this?