Can someone help me with camera stuff?

I'm creating a custom physics engine (Because it'd be easier to manipulate stuff in my own physics engine), and it all works fine, but when you go into first person perspective, the fake character (That i'm using for my engine) glitches, I'm currently using the mouse, so the part it's focused on, faces the mouse, it works, but it glitches if you go into first person perspective, and since the engine is for an fps-like game, I need it to look towards where you're looking at in FPS mode (except up/down).

I currently use this in a loop : local cf=Vector3.new(mouse.Hit.p.x,fakePlayer.CFrame.p.y,mouse.Hit.p.z)

then, when I want it to point at the mouse : fakePlayer.CFrame=CFrame.new(fakePlayer.CFrame.p,cf)

It works, but in FPS mode, it glitches out a bit... alot. Anybody have any suggestions on how to fix this?

If you need the loop code, here it is :

while wait() do
    local mouse=game:service'Players'.localPlayer:GetMouse()
    local cf=Vector3.new(mouse.Hit.p.x,fakePlayer.CFrame.p.y,mouse.Hit.p.z)
    if Keys["w"] then
        fakePlayer.CFrame=CFrame.new(fakePlayer.CFrame.p,cf)*CFrame.new(0,0,-1)
    end
    if Keys["a"] then
        fakePlayer.CFrame=CFrame.new(fakePlayer.CFrame.p,cf)*CFrame.new(-1,0,0)
    end
    if Keys["s"] then
        fakePlayer.CFrame=CFrame.new(fakePlayer.CFrame.p,cf)*CFrame.new(0,0,1)
    end
    if Keys["d"] then
        fakePlayer.CFrame=CFrame.new(fakePlayer.CFrame.p,cf)*CFrame.new(1,0,0)
    end
    if Keys["spacebar"] then
        if fakePlayer.Falling.Value==false and Jumping==false and fakePlayer and fakePlayer:findFirstChild"PhysicsScript" then
            fakePlayer.Falling.Value=true
            Jumping=true
            fakePlayer:findFirstChild"PhysicsScript".Disabled=true
            for i = 0, 1, .1 do
                local hitroof, posroof = rayCast(fakePlayer.Position, (CFrame.new(fakePlayer.Position, fakePlayer.Position + Vector3.new(0, 2, 0))).lookVector, 1, fakePlayer)
                if not hitroof then
                    fakePlayer.CFrame=fakePlayer.CFrame*CFrame.new(0, .5, 0)
                    wait()
                else
                    break
                end
            end
            fakePlayer:findFirstChild"PhysicsScript".Disabled=false
            Jumping=false
        end
    end
end