I need help with my g73jh upgrade!!

I decided to upgrade my g73jh, since my 5870 died, and bought a Dell 6990m. I did the heatsink mods that there was in the awesome post of DH48, with the collaboration of Thanatospy, @tilla, Tijo, and others (thanks, guys!!).
The laptop runs smoothly and with low temps, btw 50 and 55º (Arctic Silver mx-4). The problem comes when I play games: I decided to do a little benchmark with Crysis 2 with ultra settings and it just goes smooth (45-50 fps) but, after 3 or 4 min of gameplay, the laptop just shuts down. I tried with other games such as Skyrim and Batman: Arkham Origins, and the same issue.
It cannot be overheating because the computer does not even get warmer, and when it shuts down I can turn it on inmediately, while the usual thing is that you have to wait a few minutes for the mobo to low its temps. I do not overclock.
My theory is that it is a matter of power consumption. My power supply is the ADP-150 NB D (150 W), however it has lived better times and the battery is almost useless. In any case, the laptop has more than three years on its back and I have "squeezed" it to the maximum.
Anyone has experienced this?

Thank you so much!

 

Either your brick needs replacing or the VRMs (which are not temperature monitored directly) could be cutting out as they are not cooled properly.

 

Finally, I chose the first option, and the replacement will be this one:

Targus Universal AC Power Notebook Adapter APA05US PA 1181 08 3 Connecting Tips | eBay

It is 180w, next week I shall receive it. I really hope that this should fix this issue.
As soon as I get some news, I will let you know

 

That's the largest universal brick I have seen. Will hopefully sort it for you.

 

Yeah, it works in a lot of laptop models, including the G73JH. Probably next week I have a veredict

 

A much better option that the one hung low knock offs on ebay.