I advice to install "load applet or cpumem-applet" so you can get an eye on it most of the time.

"load applet" (Show CPU and memory usage, take screenshots, record screencasts)

"cpumem-applet" (Show CPU and memory usage in small status-area icon only)

There maybe 2 problems which i don't know which one.

1: After using wifi a battery drain accurs. (wifi switcher must eliminate that cause)
2: Some process taking a huge load on cpu, examine which one does!