Calculation rate slows to a trickle

I run a series of macros simultaneously. The macros do a
lot of copying of both formulas and values. When the macro
sequence gets to a certain point the calculation rate
begins to slow down tremendously. If I save the file and
exit Excel completely and then get back in and continue
the macro sequence where I left off, the calculation rate
is fast again. It appears as if system resources
(Memory?) are being used up as the macro is run to the
point where the calculation performance is degraded.

I'm trying to understand what's happening, and find a
Visual Basic command that I can place in between macros in
a sequence that will free up system resources as exiting
Excel appears to do.

Any help would be greatly appreciated.