The Nissens 3-row radiators can be had for around $230 if you shop around for the best deal. If the temp goes down when the heater is on, then it indicates that your thermostat is likely OK, and that you are simply not getting enough heat transfer. Causes could be worn water pump vanes not providing enough flow and clogged radiator. Be sure that A/C condenser fins are clean as well. Is the fan shroud in place (big black ABS piece that surrounds the fan)? This improves the effeciency of the fan. Also - check the viscous thermostatic fan coupling (the center hub). When hot it should be quite stiff, when cold it should let the fan feee-wheel more easily. Finally, do not discount the possibility that your gage, your temp sensor on the block, or the wiring harness is failing.

Note also that a too-lean fuel mixture will make the engine run hot.