Ford F-150 - 1997-2003 :: P0306 And P0174 Codes - Engine Overheating And Coolant Reservoir Dry

I have a 1999 4x2 F150 XL, 4.2L engine, 5-speed trans., about 197,600 miles and have been getting P0306 and P0174 codes. The check engine light came on and stayed on. Also, pretty much randomly, my engine over-heats and the engine compartment coolant reservoir is suddenly dry as a bone. I bought the truck new and this has only happened twice, during normal driving.

Clearing the 0306 and 0174 codes. Not sure what to do about the over-heating thing.. What I've done so far to try to clear the codes is the least expensive stuff:

=> Replaced both forward O2 sensors, have read where the two rear O2 sensors don't necessarily need to be replaced so I haven't replaced them yet. The sensors that came off are Bosch, as are the ones that I bought from Autozone and installed.
=> Replaced old spark plugs with Motorcraft brand, gapped to 0.054 in.
=> Replaced old spark plug wires with Motorcraft spark plug wires.
=> Cleaned the MAF sensor
=> Replaced PCV valve and replaced an section of elbow hose near it. The original elbow had a big hole right at the 90-degree bend, on the inside.
=> Replaced a cracked, dried-out, vacuum hose coming from the evap cannister purge valve and going to the front underside of the upper intake manifold.
=> replaced fuel filter.

After doing all this, I took it out on the road long enough for the computer to wake up and relearn and got the check-engine light again. Only this time it was flashing. I gather from reading in this forum that a flashing CEL is worse than a constant-on CEL so I drove back home and have yet to take it back out.

After reading about problematic ignition coils, I followed a procedure in my Haynes manual to check mine out:

=> Verified the correct resistance values at the electrical connector (0.7 ohms),
=> Verified battery voltage is present at the harness side of the electrical connector (about 12.6 volts),
=> checked the resistance values between the three sets of 'towers'. Starting from front to back, the resistance readings were 13.06 kOhms, 13.12 kOhms, and 12.98 kOhms. The correct range in the Haynes book is 6.5 to 11.5 kOhms so I thought maybe my ignition coil was bad.

I located a Motorcraft ignition coil at a local O'Reilly Auto Parts store, went there, and was able to check the resistance values on that new one. Turns out the resistance on the new one at the parts store is pretty much the same as that on the one on my truck (all three coils were 13.something kOhms), so I haven't replaced the ignition coil. I guess either my Haynes book is probably incorrect or the ignition coil on my particular F150 has different resistance values than the rest of the vehicles covered by that manual.

Next things I'm looking at replacing:

=> egr valve and the associated line to the DPFE sensor. (DPFE replaced a few years back, as a result of a P0401 code)
=> iac valve
=> upper intake plenum gaskets

I'm kind of at a loss at this point. I hate to just continue replacing things without knowing that I'm replacing the right things.