Rough cuts - help needed

I am trying to cut out some pieces to be glued together to make an octagon frame. I have a 500mm machine, with the stock 300w spindle (replaced spindle already). The stock is 2 layers of 1x aspen and I am using the .25 onsrud endmill that we purchased from Inventables. The bit is ~2" overall and has about 1.625" extending past the collet. Step down is .05, feed is 15 ipm & speed is 12,000 (whatever the standard spindle speed is with the switch on manually).

The mill has a great cut when it is moving perpendicular to the grain of the wood. it has 2 cuts that are at 22.5 degree angle to the grain where it chatters like the bit catches on an extra soft spot and the bit pulls itself into this spot. This causes the bit to flex but the machine appears to be holding its steps (position). I also need to mention the 2 cuts that cross the grain only cover about half of the bit whereas the other 2 cuts (the problem cuts) use the full width of the bit. It is also using a climb cut. I dont see how that would make a difference with the full width cuts.

Is my bit dull already? Too deep per pass? not a high eough feed rate? too slow of a spindle speed? Too much bit hanging out for the 300w spindle? I even tightened the eccentric nuts on the Z.

I have so many ideas and would like to get around these road blocks.

Thanks!!! You all have helped so much already!!!

if you can reduce the bit length below the spindle that is always best, I found that much over an inch would cause problems.

Yeah, if you have less than 1/4 of the bit secured in the chuck, you’re going to have a lot of deflection and chatter.

I’ve put the bit in as far it could go, then backed it out a little so that I could make sure the bit was tight in the collert. It just so happened that i extended down far enough that it could mill the piece out without having to flip the part. Should more of the bit be able to fit up in the spindle?

Also, haven’t found a shorter .25 bit on inventibles site.

A better question could be, how can mill something that is 1.5" thick without flipping it over?