help - Turntable playing slow

So, i just pulled my turntable out from storage and hooked it up. Put some records on, and everything sounded great. However, within a few mins it began to spin slower (quite noticeably). A couple days later same exact thing. At first it's fine, but then it slows.Any thoughts on what this might be, or how to fix it? It's a Stanton T.51 model. It's 5 years old, but has literally only been played 10 times. It's been in storage for the last couple years, and I made sure to treat it with care, so as not to damage it. Everything else works great on it.thanks everyone