BAS/ASR Warning Light & Gas Cap Warning
 

I saw a post about a BAS/ASR Warning light coming on just after starting, but mine has been coming on after I have been driving for a bit, what does this usually mean?
I have also had a lot of problems with my gas cap, and thus causing the sensor light to blink as well as the check engine light to come on. My shop told me the pressure is fine in the tank and the cap is fine, so why after all this time am I have problems with the gas cap? I sure can't seem to put it on any tighter..
Any help or opinions would be appreciated
M