Currently we are having a similar issue with several controllers (mostly 7005 running 6.4.3.1), but have seen this off and on throughout the years with 620 and 650 controllers running various flavours of 6.1 as well.
In our case, we see PPPoE disconnect every 3-4 minutes, and then re-connect. I have seen the same Max LCP echo requests message in our logs as well, although usually we get a LCP TERMREQ error instead.
I have done a packet capture of the uplink, and in our case, what we are seeing is a failure to reply to PPP LCP echo requests sent by the ISP, and the failure of the controller to respond to these causing the ISP to close the connection.
I just thought I would mention it, as we are currently working with Aruba TAC to try and find a solution to our issue, so if anything comes out of it, I will try and remember to post the results in case it might help your situatiion as well :)