motor / esc problem

[Andy68]

I have a problem with one or possibly both thet are

Matrix evolution spec racing esc

Speed passion 13.5/27t motor (brushless)

Problem goes like this , full power achieved then after about 10 seconds motor slows to approx 25% gradually over about another 10 seconds , esc and motor and wiring between get hot , wiring from battery to esc ok Motor also runs a bit lumpy ? like a misfire
Verdicts please guys , thanks for taking the time to read this
BTW Both items second-hand

Andy