Current-carrying compression connections, among other applications widely used to join transmission line conductors, are important components that fulfill crucial functions in an overhead line. In practical application, electric-thermal, mechanical and ambient loads act on the connections and influence their long-term behavior depending on the initial joint resistance. Recently, thermal irregularities of current carrying connections are increasingly diagnosed especially for high-utilized overhead lines. In particular, the interaction of relevant aging mechanisms and deterioration mechanisms that accelerate reaching the end of lifetime is still not fully understood. In order to determine factors that can limit the service life of compression connections, long-term tests for several thousand hours were carried out under realistic outdoor operating conditions. Electric-thermal and mechanical loads were therefore applied to the tested connections. The resistance behavior and the thermal operating behavior of typical compression fittings were evaluated. It was found that mechanical tensile loads can deteriorate compression connections, especially if they were made with service-aged transmission line conductors. Connections made with the same connectors and brand-new conductors were not affected. Hence, the paper summarizes the results and the found correlations for unstable thermal operating behavior of compression connections and presents some recommendations for practice, diagnosing and testing.