Unless your switch contains a neutral connection, power for the switch electronics is fed through the load. While small, that can be enough to cause sort of a "relaxation oscillator" with the bulb electronics, and cause the effect you see.
The 3-wire switches designed for CFL and LED loads feed the switch power through neutral rather than the load. The relay switches do supply a very small current through the load to sense which state the relay is in, but that should not cause the bulb to pulsate like yours is doing.
None of the XTB signal boosters would make any difference in this issue.