yeah i started gettin that problem too with the PO recently. i use a dell inspiron 6000 with an intel centrino processor (insert intel jingle here). ive a feeling its down to the processor getting warmer and that slight increase in temperature really affects the performance after a few mins. and like the last post bout saving and then quit the game, its my only choice really. dont really think its down to the texture sizes purely, though they would affect how long the game takes to load up each track.
however, i find i get it when i play f1 challenge 99 02 sometimes too, so clearly its not just a problem with grand prix 4 alone. its probably a hardware issue really, sadly.