I run GP3 on a Hercules Prophet Kyro II 4500 64MB, and yes, it is a bit stop/start, although not too bad. The game will pause for a second or two about 1 - 2 times per lap on average, which can be v. annoying
. Also, the game will run slowly at the start of a race, when there are lots of cars onscreen. Apart from that, the graphics are good.
Try turning anti-alias 4x4 off, and running on a lesser detail level.
Evidently, these problems will be solved in GP4. Here's hoping...