Fps problem.

hey people,

i have this fps problem on that game team fortress (not 2 the classic one, it is almost 15 years old). ive just bought my pc and its kinda impossible to have that kind of fps problem on this game i think. sometimes its 99-100 (which it actaully suppose to be) and sometimes it drops to 60-70-80 or 30 depends where i am aiming at. wanted to ask if you guys know the solution.

thanks,