Cooling Guru Needed !!

[Evil Joe]

89 302 auto w/ 126k miles
no major modifications (K&N, 3:55, mufflers, timing advanced to 12 degrees but I always use 91 octane)
note: heating core is bypassed (damn thing leaks)

Temperature guage was reading about 220 F. So, I:
1.) Flushed the engine block with water.
2.) Changed thermostat to 180 F.
3.) Replaced coolant (50:50 mix).
4.) No leaks.

This didn't help. Guage was still reading 220 F. So, I:
1.) Replaced radiator cap.
2.) Replaced temperature sending unit.
3.) No leaks.

Guage read 200 F. Definite improvement, but it should read 180 F. Next, I measured the resistance of the temperature sending unit over a range of temperatures. Now, Ford says the unit ranges from 10 ohms (full, or 270 F) to 73 ohms (empty, or 130 F) which, assuming that scales linearly, allows one to calculate the expected temperature. Here are my results:

Resistance (ohms) Guage (F) Expected Temp (F)
65 150 148
48 180 186
43 185 197
42 195 199
41 200 201
36 205 215

These results tell me two things. 1.) My car is running hot, FOR REAL. 2.) My guage is slightly off. (Also, my guage always showed some sort of erradic behavior - bouncing from 160 F to 220 F to 195 F to all over the place - especially after driving on the highway or flooring the accelerator).

Anyways, I need some help on what to do next (before I buy a new radiator). Here are some specific questions:

1.) Does the temperature sending unit's resistance scale linearly with resistance?
2.) Is there some trade secret to changing coolant? (Like clever ways to purge air?) I just drained the system, flushed, and then filled it back up with coolant.
3.) Is it worth scouting for a new guage for my instrument cluster?
4.) WHY IS MY CAR STILL RUNNING HOT!?

Thanks for all your input. I would really, REALLY like to solve this problem. It is driving me insane.

Evil Joe