2000 Olds Silhouette Heating/Cooling Problem

Primary problem is that the temp gauge gets too hot and engine will overheat. The temp gauge will rise to hot and then lower to cold (below normal temp) and then repeat while at idle or driving at low or high speed. The heater mostly blows cold air until temp gauge gets really hot. The cooling fans don't come on until temp gauge is at the 3/4 mark (instead of the normal temp of 1/4 mark). The low coolant light occassionally comes on but the overflow resouvoir and radiator are full. Finally, there are various other issues that may or may not be related but I will list them in case there is a correlation: pulsing headlights/dash lights (like alternator is bad), air bag light sporadically on, driver's electric seat heater doesn't work.