Default s4 wireless charging intermittently

I bought a QI wireless charging adapter from Ebay and installed it in my s4. I used Nokia QI wireless charging pad. However, I found my the charging was intermittent after like 5min of continuous charging and that was when the Nokia pad & my s4 both got very warm.. The phenomenon was: the screen kept coming on every 5seconds like the phone was disconnected from the power source and reconncted.

I found that, when I put s4 off from the charging pad, let phone cool down, put it back, it would work flawlessly for 5min, again charging intermittently. This bothers me, as apparently the physical charging worked for my s4. Might be the wirless charging issue related to s4 firmware?

Any inputs are welcomed.