Iphone 5 acting strange lately. 2 major issues.

Question

Hello all.  First time poster here. 
About a year ago I got my first Apple Iphone.  I was an Android proponent prior to that, but had numerous issues with the one application I use the most frequently which is Yahoo Messenger, so I decided to try an iphone5 and see if it would be an improvement.  I use yahoo to speak with a close friend of mine who is overseas, because she refuses to use anything else.  Personally I freaking hate the program, but she is too set in her ways to change.  The iphone has been an improvement over my old droid bionic, but lately it has been getting a little weird.
The primary issue I am having is the fact that yahoo messenger will no longer notify when there is a new message if the screen is closed, either audibly or through vibration.  This means I must actively go into messenger and open my contacts to see if anyone sent me a message, which is unnacceptable.  There has been no update to messenger in what sems like years, and probably is close to it.  Any ideas on what causes that or how to fix it?
The other issue I have had is random screen dimming.  This issue is less frequent, but intensely irritating.  Basically, the phone will suddenly decide that the only appropriate screen brightness is full dim, and the only way to fix it is to go into the settings and move the brightness slider.  Even rebooting the phone will not correct the issue.  The slider will actually show in the exact position where you left it, but the instant you touch it the brightness returns to normal.  Of course when it is bright outside, you cannot actually see anything on the screen to find the settings, and this problem seems to occur most frequently when using navigation on a bright sunny day, rendering the phone and navigation useless until you find a place to pull over and fumble with the phone long enough to fix it.  It got me lost in NYC traffic on a recent trip, and cost me over 45 minutes worth of detours etc.  I nearly threw the **** phone out the window permanently, and nearby sailors were swooning in disbelief at my language on that day. 
Any pointers on how to address this situation folks?
V/R
Alan

Answer 1

Try this:
Go into a dimly lit or dark room.
Launch the Settings app from the Home screen of your iPhone, iPad, or iPod touch.
Tap on Brightness & Wallpaper.
Turn the auto-brightness setting to Off.
Adjust the brightness slider to the level you would like your screen brightness to be at in the given environment. For instance, if you're in a completely dark room, you'll probably want to put the slider all the way to low or pretty close to all the way down.
Turn the auto-brightness setting to On again.
Now flip on a light switch.
Watch the brightness slider move upwards to adjust to the new light entering the sensor.
As long as the brightness slider is moving on its own, you have successfully re-calibrated the sensor and it should work properly from here on out.