1999 Silverado transmission

I have a 1999 Silverado 1500 with the
5.3 v8 and just under 215,000 miles.
Occasionally when the truck has
been sitting for a few hours or
overnight it will hesitate. It's like the
power drops out, it won't accelerate
and when it finally does shift it is
extremely hard and sometimes even
makes a clunking noise. After the
initial hesitation and shift it runs like
new with no issues. I have very little
knowledge about transmissions, but
that is the only thing I can think of
that would cause that problem. I
need to know if this is going to be a
major, expensive fix or if it's just
some small thing. I love this truck,
but with the year and mileage I can't
see putting a lot of money into major
repairs. Any advise would be
welcome. Thank you.