trouble with travel speeds

[gibl5s]

I have been making profile cuts in 1/2" MDF with a .125 end mill bit. I've been making hundreds of cuts and all of a sudden while making the cut it slows way down kinda like it would when it comes to a tab but it is NO tabs. Is there something that needs to be reset? I am making 3 passes @ 3ips. has worked fine. now it is very inconsistent, (not smooth). takes a bit longer to cut the entire file.
Has anyone ever had this trouble before?
thanks in advance