I believe this is the camera you have http://www.ivigilcctv.net/specsh/TD20_TD22HP_TD21.pdf
Based on the spec sheet it draws 390ma max. Assuming the cable installed for power is 18AWG (you will need to check this) the maximum rated distance from the power supply is calculated below.
Distance Calculator
Power Supply Output Voltage
12 volts
Camera Minimum Operating Voltage (+/- 10% is what your camera is rated)
10.8 volts
Camera Current Consumption
390 milliamps
Camera Power Consumption
4.2 watts
Number of Power Wire Pairs
1 pairs
Loop Resistance for 1 pair
42.4 ohms
Wire Gauge
18 AWG
Maximum Wire Temperature
122° F
50° C
Maximum Distance
185 feet (power supply to camera)
56 meters
Battery calcs above taken from http://www.nvt.com/ showing the max distance this camera should be. Your problems described very much sound like a power issue. Remember is it not just the "as the crow flies" distance to your camera, its the total length of the cable including the "up, down and around" route your cable was installed.
As mentioned by others, any plug in transformer/power supply that gives you 12vdc and at least 500ma plugged in closer to the camera should resolve the issue. Alternatively, you can run a larger power cable from the power supply out to the camera. If you decide to expand or upgrade this system in the future, reach out to a local professional. They will design the system to account for these types of issues.
Hope this helps-
Jesse