2 things that will cause this.
one is the shifting cables arnt anchored anymore, most likely in the engine bay, get someone to smash it through the sprockets as you watch the cables in the engine bay. If the outside of the cable moves then re anchor it and you will be laughing.
if its not that check the shifter bushes, there is the metal ring, then rubber on the inside, then brass sheath on the inside. Then that slides over on a pin on the linkage on the gearbox. The rubber gets soft and the brass wears oval (insted of round obviously)
Racing beat make bushes for the shifters, which i would assume are the same as the gtfour. The 205's have two equal sized bushes and the 185 have one large and one small. Might be harder to get to them with your steering arms and brake booster etc on the wrong side of the engine bay
hehe.
EDIT: i made my own solid bush from bronze impregnated teflon and did away with the rubber and brass sheath altogther. The differance is crazy, not harsh at all.
do it...do it now.