VHDL UART how to handle incoming bytes?

Hi,

I'm trying to build a counter design using VHDL and having trouble about 
UART RX link.

I want to transmit some commands from computer like start, stop, reset 
and counting period. However, using case statement I could't succeed to 
process my commands. Because after sending period info when I send start 
command, period turns to another value. I expect it to stay the same 
since I didn't send any command regarding to period.

Thus I'm thinking, this is not the correct way to handle incoming bytes. 
Then the question is how it must be? What kind of a structure I must 
construct?

p.s If you would like to see, I have also uploaded related design files.

Any idea is appreciated...
Thanks in advance