Sputtering and Shaking when vehicle is turned on in 2000 Jeep Grand Cherokee Laredo?

I have had radiator issues in the past
and know I need to replace it again, I
always check my gauges and make
sure my coolant isn't running low. So
today after driving it around a lot for
a bunch of errands, I stopped
somewhere for a few hours, let it
cool down, when I went to turn it on
it sputtered and shook for a few
seconds and then slowly returned to
normal, it then drive fine with no
problems. I left and went somewhere
else just down the road and 3 hours
later when I left it started up the
same way with the sputter and shake
but it felt and sounded worse than
the first time, after a few seconds it
slowly returned to normal and drive
fine again. Any ideas what it is?
Need help ASAP. Please and thank
you!