Server: The Operation Has Been Cancelled. - When Processing Cube

When I highlight a few
partitions and start processing, the process occasionally stops with a
message that operation has been canceled, like this:

Response 3
Server: the operation has been cancelled.
Response 4

Server: the operation has been cancelled.

Response 5
Server: the operation has been cancelled.

..etc...

(no further error message details are provided)
SQL profiler shows that batch was completed (but rowcount shown in process progress log is too small).

Analysis
Services profiler shows no messages at that time at all. It just shows
messages when it started, and when I restarted the processing.

The Analysis trace appears to be stopped when processing stops with "Server: the operation has been cancelled."This
is an occasional error and sometime occurs within 15-20 minutes from
starting to process. It could fail on 1st partition in the process list
, or on some partition in the middle. Some partitions might run for a
few hours and not error out, but sometimes it fails quickly.I was not sure if this is an issue with the underlying fact data, so I broke up the last partition where it failed into 5 smaller partitions, they processed OK separately.I think restarting SQL Server and SSAS helps to process a few more partitions. Some that failed with this problem process OK after restart, but some fail again.
(apologies for re-posting, wanted to put under more specific thread title)