That model has a very small power supply. The GT 710 requires a psu of at least 300w as does the GT 750. Amazing that you got it to work. You should mention which version of the 755 you have. There are 4! All have small psu's. 

See this thread-- https://forums.tomshardware.com/threads/power-supplies-that-work-with-a-dell-optiplex-755.1882815/

See the post from Mary G.

I have the desktop optiplex 755

he means what form factor is your PC 755 comes in 4 forms, at least.

MT and USFF are not the same. pC. nor same Motherboards etc...

I fact tell helper 3 things, besides gtx model full GTX partnumbers on tag

1: what OS. w10 64bit or say VISTA 32.?

2: what  form factor

3: what PSU model or watts stamped on side, 200w or 460 or  ? what? some PC have 3 choices new.

NO OS stated at all nor case type.(chassis form factor)

w10--64bit?

gt210 ,circa 2009  30watts relic card.

GT710 7 years newer, 2 times faster core but only 19watts,  my guess is you failed to uninstall the 210 driver before installing the 710, am I right.

or failed to use  the newest  driver from nvidia.com  for unstated OS?

an old GTX750 is a better old card, in fact best for speed best for price and only 60watts.

going higher will need more PSU, in most cases but  LOL no case stated (the case of the missing case) lOL
How much ram do you have,,.  2 GB or 16?

is the OS freshly loaded or full of infections, endless, ….

if running HDD defrag it and run ccleaner and google how to run SFC and DISM.

but do the nvidia drivers,. for non dell cards.

the old 710 GeForce ended at w8.1-64bit. but may run on w10, try them.

click series 7 here

https://www.geforce.com/drivers

these are usually the best there are, with old GPU card from them.