Baby lock serger motor slows down after sewing 20 minutes. Why?

Call your local service shop. Baby Locks in closets for years usually have this problem The manual recommends annual maintenance by a pro, and they will oil the internal lobes of the feed dog apparatus. It cannot be oiled from the outside, nor with covers off, but must be taken apart. If those lobes lose the oil and heat up you will feel "surging" then slowing repeatedly -- fast then slow then fast etc -- until the lobes over heat and sort of weld themselves to that small barrel looking case. So a Baby Lock can sew, then temporarily be sluggish then later slow till sluggish; and at worst, seem to be doing everything right BUT not advancing the cloth -- that's when the worst damage is done and professional repair is needed. In PA might cost $150 to fix. $70 in parts; $80 in labor (2012 prices)
Been there, done that.