To buy a new graphics card would be a total waist of money.
Audio production programs dont need that sorta power cause hence , they dont use much, barely any graphics. Most graphics intensity you might seen in audio production is the DB meter bouncing up and down or the song scrolling left to right.
If you was using Nuendo AND you was doing video production then there would be a big difference and you would need a good, decent card to handle the video graphics, but still not audio.

CPU (Central Processing Unit , Main computer processor)

GPU (Graphics Processing Unit, Video Card )
APU ( Audio Processing Unit, Audio Card or onboard Audio card aka integrated audio card on the motherboard)
Those last 2 devices has it's own "Brain", and offloads the work the main cpu would have to do if those devices didn't exsist.

If you think it's memory, a really good program also to use is memtest86.
Run it before you goto work, or bed for the night/day. usually the longer the better. Days if you can.

It almost sounds like your CPU is straining. Do you have other apps running in the background you dont know about?
Maybe antivirus or anything?
I use to run stupid stuff like my tv card while messing around a bit with FL producer and my sound would jerk , when i moved windows it would jerk more, sorta like you mentioned. When i closed out the tv card program it would stop.