dell 3542 0.79ghz problem

l have a problem with cpu speed

cpu speed always staying on 0,79 ghz and l dont know what can l do .

my laptop cpu intel i7 4510U 2-3.1ghz 

l am using laptop on high performance mode 

it is becoming faster  with throttlestop and speed arriving maximum 2.9 ghz. 

l also opened laptop with portable windows 8 but it is working same . 

and last week l buy a 2nd hand orginal battery . batterry was full so when l use it with only battery performans becoming normal . but when l use my charger orginal charger .it was becoming 0,79  after l use also a different charger but it is same .