You apparently have a cheap noname psu that does not deliver sufficient power at 12v where you need it.
Do not believe 750W.
If it does not have a 8 pin pcie power lead, it does not have sufficient capability.
Past that, it is dangerous.
A cheap PSU will be made of substandard components. It will not have safety and overload protections.
The danger is if it fails under load, it can destroy anything it is connected to.
It will deliver advertised power only at room temperatures, not at higher temperatures found when installed in a case.
The wattage will be delivered on the 3 and 5v rails, not on the 12v rails where modern parts
like the CPU and Graphics cards need it. What power is delivered may fluctuate and cause instability
issues that are hard to diagnose.
The fan will need to spin up higher to cool it, making it noisy.
A cheap PSU can become very expensive.
Run with the gt210 until you can buy a decent quality psu.