I've always found full tilt a bit laggy,
but better than other sites with blocky graphics (looking like 8 bit 1980s micros) which can be even more laggy.
Sometimes I can hear the beeps for action but no cards are displayed.
appears to have the most reliable client software, and is probably the only software that could handle the world record attempts. I remember fulltilt lagging badly when they tried to do big field tournaments.
Usually though the problem is an over worked computer and poor software or operating system that doesn't release memory properly when tasks have ended.
Restarting the computer on synchronised breaks often helped.
Disconnects could be caused by a poor Internet Service Provider, or a badly performing modem router (modem reset can help), or a problem with the client device.
From your description though it sounds like time to externally backup all data and make a new clean operating system install, obviously followed by service pack updates, and application software and data reloading.