Frame rate problem

[Poulp']

At new year, I installed EB 1.2 on a blank HDD.
Everything went smoothly, no problem installing EB (plus force diplomacy minimod) and the game runs fine; however, if I reload or quit and re enter the game, the frame rate goes wrong. It alternates between a normal rate and sometimes speeds up for a second or two and then goes back to normal.

Here's my specs:
AMD Athlon 64 X2 Dual Core Processor 4600+
2GB RAM
Windows XP SP3
NVIDIA GeForce 8600 GT