HL2 Old Engine stuttering

for some odd reason the game stutters and freezes terribly around certain moments in Route Kanal, like the part where one of the CP throws a burning barrel down the stairs and other areas like when they throw more barrels into the water with all the wooden platforms. how do i fix this?
the game also did crash for me at one point but i dont remember how, where and when lol my memory isnt great

2 months later I realize
the autosaving might be the cause for the weird stutters/lag at specfic areas