My van is not doing well, maybe someone can help,

What happens is it feels as if I am driving into a head wind. When I slow
down in third or fourth gear it will buck seems to be running lean, by
putting in the clutch and then letting it out it will stop bucking and run
normal.
I thought it was
the temp sensor, got a new one and it seemed to run fine the idle
dropped 200 rpms and it seemed smooth. I was happy but then over the
weekend I went to Portland and started to notice as if I was driving
into a head wind. The next morining I went to Boston and it seemed to be
running good. About a half hour into the trip it again started to feel
as if I was in a strong head wind. I stopped for about a half hour and
it was fine when I started out again. After about  a 1/2 hour it started
running bad again. I check all the connections but could not find
anything wrong. Started back on Monday and it started all over after
about a 1/2 hour. Got back and put the old sensor in and it does run
different but not right. Just to let you know I tried swapping the
computer, air flow meter, and what ever else I could do. I have  a new
air filter the plugs are okay and the wires and distributor cap and
rotor are in good condition. When I  start it up it will run okay until
it warms up and then it is not running at 100%.
My gas mileage has dropped about 2 mpg (18.5 from 20.5).

Any ideas?
Thanks

Gary Domestico   W1JRW
Greatneck Computer Repair
369 Main Street
Rockland, Maine 04841
207- 594-0983