When I plug in a portable heater, the fan will work but does not heat on any outlet on that particular circuit... the voltage drops to 15 volts. I have changed all wall outets on that circuit but I continue to have same results. But I can plug the heater into an outlet on another circuit and it works great. I've been told that I have a high resistance in that circuit... what causes the high resistance and how do you find where it is? Thanks in advance!

You need to determine all the recepticles on the bad circuit.
Measure between Hot and ground with the heater, if it drops to 15 Volts, the HOT is likely the problem, if it doesn't drop, more likely the neutral and could be at Panel, or one of the recepticles closest to the panel, on that circuit.

Jten,

Too much resistance is caused by to many active devices on the line at the same time. What else is connected to this circuit? Does this circuit ever trip the circuit breaker?

It is likely a loose hot or neutral, if their was too much load it should trip breaker.
The reading between Hot and ground under load will determine which.
Also Check tightness on breaker, breaker on buss connection, and neutral connection in panel.