lag and stutter in games

[a.j]

pc specs:
core i5 4460
asus b85m mb
asus gtx 780 3gb
8gb ram
240 gb ssd
1tb hdd
600w evga power supply 
so i sold this pc to my friend and before i sell it he checked it if it has any issues we ran csgo max settings 300 fps and arma 3 60 fps sometimes higher and sometimes lower and we ran heaven bechmark ultra settings x8 msaa FHD and theres one setting that set to normal and fps ranges are from 50-100 and even higher so he called me today and said that he cant run gta iv on low without lag and his fps was 16-40 note that gta iv is a pirated edition and he ran cs 1.6 and he told the same and he told me that he might connected the vga to the wrong dvi adapter and maybe because his screen is old 19 inch screen so what causes these issues 
i have the latest drivers 

[ForFour]

gta 4 needs to be patched to the newest version its a problem with the game. don't know about cs 1.6 tho.

[GeorgeKellow]

Try reseat the GPU 

[a.j]

does the screen affect anything it might have a low refresh rate because we tested on my screen and it had no issues and maybe he connected to dvi i not dvi d maybe that affect becasue im sure that my had no isseus