Carving changes after 3rd pass

I’m trying to cut out some letters that at 6" x “8 into mdf. its 3/4” material and I’m cutting right through. I’ve used a 1/4" compression bit and a 1/8" downcut bit. each time i get 2 good passes then we go off path. I don’t have agressive cut feed rates or depth rates. What is going on?

Without more detail, I’d guess that you might be packing MDF into the cut.
Have you tried an upcut?
What are your feeds and speeds and depth of cut?