paste special/values can be very slow
 

Using VBA, I have a script that keeps adding a row of data to one worksheet,
and a row of forumulaes to a second worksheet that analysis the data. the
data is "real time". to keep the size of the worksheet with formula
reasonable, I copy/paste special the values of each old row onto itself every
time there is a new row of formula created. I use range.calculate to
calculate the new formulas. The problem is that yptically the copy/paste
special takes a few msec, but every once in a while (about 5% of the time)
the past special can take seconds.

I think I've done all the usual things like make sure there are no volatile
functions and turn of automatic recalc and screen updating, as if evidenced
by the fact that the paste only takes a few millisecond most of the time.

Any suggestions as to what I should look at?