Nokia :: N900 Running Slow - Phone Freezes While An Sms Or Email Is Coming Through

For no real reason my phone has got sluggish with certain tasks, like the phone will freeze while an sms or email is coming through, sometimes when opening an sms i get a blank screen where the writing should be for a few seconds and general sluggishness.

On previous symbian devices this was largely solved by freeing up some phone memory, is it the same on the n900? df -h / in x term says 71% free. or is this normal for the phone, given i flashed it recently and that apparently made no difference