It's most likely the stabilizer on the left shift key. Just throw some silicone grease on it (on the metal bar and inside the white plastic inserts). That should fix it.
You can get silicone grease at Home Depot, Lowes, Ace Hardware, etc...in the plumbing department for around $3-4.
You are going to have to take the keycap off the keyboard to lube the stabilizer though.