Turn your phone on. Press the power button to lock the screen. Press the home button to wake up the phone. Keep repeating this process over and over.
If the screen reliably locks every time you press the button, then the power button is reliable. If it does not, your issue entering DFU is that your power button is not reliable enough to be held down during the procedure.
If the screen reliably wakes every time you press the button, then the home button is reliable. If it does not, your issue entering DFU is that your home button is not reliable enough to be held down during the procedure.
If either of your buttons is unreliable, you will need to fix that hardware problem first before attempting to get the phone into DFU.