'93 Ford Escort LX Engine Rumbles When First Started

The engine of my 1993 Ford Escort LX station wagon rumbles and shakes when I first
start it. Sometimes I have to rev it a little to get it to level out.It always goes away after
10 to 20 seconds with gently revving the engine, Sometimes if it has sat a bit it will jerk
for the first mile or so of driving. Once it is warmed up it runs fine and does not have not
problems. And if it has been driven earlier in the day, when started again the motor
does not shake. It is only when it has sat for 8 hours. The longer it has sat the worse
the problem is. Once warmed up it does not stall or die, and runs perfectly fine. The
problem is only when I start it after it has sat a while. The plugs and spark plug wires
were changed two years ago. It gets regular oil changes and does not leak or burn oil. I
have been putting Heet in every other full tank thinking it was water in the gas, and
perhaps water was condensing in the cylinders, and was being burned off when it has
run a bit, but this does not seem to help. It reminds me of starting a very old truck or
tractor on a very cold morning. Any ideas what it could be? I do not want to put it in the
shop if it is an easy fix I can do myself. I am thinking it is something minor as it is only
a problem at start up.