Ootake speeds up and slows down

Hi, I'm having a strange problem with Ootake whilst playing Ginga Fukei Densetsu. It's the only PCE game I've ever played so I don't know if the problem is unique to this game or not.

Anyway I'll be playing fine for a few minutes then the game will slow down to about half speed, the music stays playing at it's normal rate.

After another few minutes of playing like this the game then speeds up and plays perfectly (maybe even a little too fast at first).

The games speeds just keeps bouncing up then down constantly every few minutes.

does anyone know what might be causing this?

I am running windows xp 32bit (tinyxp) on an asus laptop.

Any help appreciated.

EDIT: Sorry if this is in the wrong section, I'm not sure where it should go.