FS2004 Keeps Jolting Even With Good Frames?

Hello all,

was wondering if someone could help me?

i have recently changed computer and installed FS2004 onto it and i am getting frames of around 40fps however when i am taxiing to the runway in the cockpit/virtual cockpit, it keeps jolting, not the aircraft but the system?

When i go outside to Spot view i get frames of around 10-20 and there is no jolts?

By jolts i mean a very slight pause every sort of 2 seconds? Which gets very annoying...

Apologies if this is a basic question,
Any ideas or guidance would be great!

Thanks,

James