Many applications in organic and printed electronic circuits require the implementation of resistors. For this it is essential to identify potential resistive materials, the processes and methods to structure them, but also to analyze their resistive properties on flexible substrates. One of the most important issues of resistors properties is the long-term stability under thermal, mechanical and electrical stresses. These are the main factors that have a major contribution on the drift of the resistor value. Especially for flexible components mechanical influences, like bending, are a new important issue for reliability, which is mostly unknown from rigid substrates. With highly isolating flexible substrates the devices become also susceptible to electrical stresses. We analyzed a polymer resistive material, manufactured in a reel-to-reel screen printing technology on plastic films. The stability of the resistive properties was investigated by different reliability tests, like temperature storage, temperature cycling, temperature-humidity stress, mechanical bending and electrical stress. The measurements show that the thick film flexible, carbon based polymer resistors are mostly influenced by humidity and temperature. Bending shows a surprising low influence on the investigated materials. All investigated resistors are susceptible to high energy pulses.