Wait?? You are asking questions about a problem that just started on your PC after you added a new GPU, and a new monitor. You have eliminated the monitor as the problem, correct? So what changed? Only the GPU correct. So, have your professional test the gpu in another PC- if it works then there is no doubt it is your PSU. Or throw in your old GPU and see if the problem persists= then the problem is the new GPU.
On the other hand- you could just take the advice of a guy (PP) who had helped many, many people on this site and realize a 5 year old no-name original PSU that is either "400 or 450 watts" may be the problem. It was pointed out very early in this thread that the GPU requires 500 watts- remember. The new GPU needs more power that the old one - therefore, the new GPU and the CPU/mobo all fail to get good power and "blam"= problems. The real question is - did you damage components by using an underpowered PSU or not? So now- because the PSU could have damaged the GPU - you have a permanently damaged GPU so even if you switch out the PSU - the problem will persist. If there is anything I have learned by trolling this site is that the one thing you cannot overlook when putting together a PC is a proper PSU for the system you are building or in this case the system you are changing.