At my wit's end! Copy+Paste and Saving issues with MC 4

Anyone have any suggestions or known fixes for the following:
Whenever I do extensive MIDI editing to a drum track in the Piano Roll in MC 4 it seems like something is getting "full" (maybe memory... I don't know) . I am able to make some edits, but if I try to edit "too much" (I don't know where the limit is)  after a while MC 4 can't seem to copy+paste a MIDI track and saving takes quite a long time (several minutes).
 
An example: I have a song file. I used EZD to create the basic drum tracks by copying in the grooves, etc. Then I go back in and edit the MIDI notes in the piano roll using the pencil to add in fills, cymbals, etc.  I am running both Roland and EZD drum kits through the synth rack so I have two MIDI tracks in the track view for playback (Tracks 1 and 4). When I am finished editing the drums in the piano roll, I switch back to the track view and copy the entire MIDI track from track 1 (Roland) to track 4 (EZD).  This works just fine the first few times I copy+paste, But, as I continue editing, at some point, MC has decided I've given it too much information and starts to move VERY slowly. Meaning: Copying Track 1 to Track 4 (I use Ctrl+C and then Ctrl+v to move the track) takes SEVERAL MINUTES. It doesn't happen when I first open the file. It seems to accumulate during the session until I reach that magic tipping point where MC just seems to have a hard time with it. I have sort of found a way to work around this by copying and pasting a very short segment first. Then, if I copy the whole midi track it generally pastes OK. However, it doesn't always work and saving still takes a long time (several minutes as well.)
 
Things I have tried:
Defrag hard drive
Delete extra files (I have about 50% free hard drive space)
Saving MC files to different folder
Opening/Closing MC
 
My System setup:
Music Creator 4
EZ Drummer
Win XP
Not sure what audio drivers or other "stuff" I have.
Adjusted latency/buffer
 
I am not sure what is causing this but it would be great to find out. It kills my productivity. It sometimes takes 20 minutes or more just to get through editing one or two midi notes and then saving the file so I don't lose my work.
 
To clarify: I have used MC 4 for almost 7 years and have done many songs this way. It's only in the last few months that I have seen this behavior. Also, I have a friend who sends me MIDI tracks. If I copy and paste from his file into my file, there are no issues. I have only noticed this specific issue when I do manual editing in the piano roll.
 
Thanks for any help!

What kind of PC: Processor type and amount of RAM? What kind of HDDs?

MIDI tracks are just data, very very low amount of data.  much lower than audio tracks.  it should take only millisecs to perform a cut and paste of part of MIDI track.
 
if this started happening in the last few months then you have to look at what has happened in the last few months to your computer, like Herb is suggesting.  I would also suggest that you look at possible viruses and especially malware.  malware could easily be the culprit.  you might have something recording your actions on the computer and sending it somewhere, or trying to.

Check the sample rates. 44.1khz and 24 bits for recording should be standard.

shut off the auto save function.
 
Do all saving manually. I save after any new track that I like, and also after editing and at regular intervals while editing.
 
Cake will remind you to save if you try to exit and have not saved it yet..... but always save as I mentioned above. You never know when windows will decide to "act up" and crash.