When did this first happen? Did it happen after you installed something? Did it happen after something was updated? What firmware are you on? Was this an immediate change, intermittent then constant, or slowly change? Have you restarted your device? Have you cleared any possible RAM cluttering?
i dont know if im contributing but i have a similar problem. cydia works fine except when i try to put in sources for roms etc. it just stops at " downloading packages" and i have to restart cydia a couple times. even after that it keeps trying to download the source