This script works sometimes, and other times doesn't?

You don't really have to use all of these returns and such. You really don't need to check if the hit part has a parent either. Instead, lets just start our touch event first.

script.Parent.Touched:connect(function(hit)
	-- ...
end)

Now, we should use a Debounce because lets not spam this script's activation.

debounce = false -- You can change the variable to anything! (I use db)

script.Parent.Touched:connect(function(hit)
	if debounce == false then db = true --Instead of two lines, this shrinks the script, doing the same
		-- ...
	end
end)

Now we will check if an actual player touched it.

debounce = false -- You can change the variable to anything! (I use db)

script.Parent.Touched:connect(function(hit)
	if debounce == false then db = true --Instead of two lines, this shrinks the script, doing the same
		if hit.Parent:FindFirstChild("Humanoid") then
		-- ...
		end
	end
end)

Now, lets just add all of your code into this script.

debounce = false -- You can change the variable to anything! (I use db)

script.Parent.Touched:connect(function(hit)
	if debounce == false then db = true --Instead of two lines, this shrinks the script, doing the same
		local h = hit.Parent:FindFirstChild("Humanoid")
		if h then
			h.WalkSpeed = 16 * 2
			hit.Parent.Shirt.ShirtTemplate = "http://www.roblox.com/asset/?id=11404364"
			hit.Parent.Pants.PantsTemplate = "http://www.roblox.com/asset/?id=253433738"
		end
	end
	wait(1) -- This script will work again in 1 second
	db = false
end)

Above is the finished script.

• If the player doesn't have a Shirt/Pants on, this will not work!