Unless you have an App that is corrupting the OS it sounds like drop damage or water damage. Ever have it in a room with high moisture or steam? Wet from rain?

If it is water and you know it was recent then try the phone in a bag of rice for a couple of days but it may not help. If you did drop it, it could have keyboard and headset connector damage.

Depending on your ability, parts are available but repair may be beyond you. If it's old enough it may be better to upgrade.