As others have said, you need to increase your feed rate or decrease your spindle speed. I have used the Onsrud O-flute bits with excellent results with acrylic. I don't have the exact part numbers, but I use the cutters they recommend for hard plastic. With rigidity issues or material lifting you need to decrease your RPM as opposed to increasing your feed rate.
Take a look at the Onsrud web site. They give the recommended chip load and the formulas on how to find feed rate for a given RPM. Also, if you are not familiar with how chip loads work go to
www.cnccookbook.com and read some of their articles and download the free G-wizard demo.
You can also try the Onsrud down spiral cutters if you can't keep your material down. What I do is a test cut of the spoil board to cut about 1/8" or more into the spoilboard first then tape down the material. The cut allows you to not have tape right at the cuts since it gums up the cutter and makes chips stick. It also gives a relief in the spoil board for the downward moving chips to go into. I also use a shopvac to vacuum up the chips as they are being made.
Now that I have a laser I hardly ever cut acrylic on the router or mill any more. Edges come out polished and no chance of cracking the material. A 40W laser will do 1/4" acrylic like butter.