fuel trouble

What chassis? Depending on how old the rig is, you may or may not have a throttle cable. Same with the OBD code, they only applied to vehicles 96 and newer. You would be getting a check engine light to indicate there was a fault code.

OBd code ? never tryed to see that have 1400 miles to go and thru Atlanta,and Chattanooga Sunday. how do I check the code

Freightliner ,340 cum after this happen today just drove at 45 mph about 2 miles and then ran fine, Transmission did try to shift it self a few times,did put it back on cruise and made no difference. Last time this happen I changed fuel filter it only had about half full of fuel refilled it put it back on and it has been ok for 500 miles before this again. Air filter good its like fuel filter runs low on fuel.

This is most likely an electronic problem, not a fuel problem. Sounds like you're going into limp home mode.

Often this is due to a faulty sensor or wiring going to a sensor.

What are the OBD codes it's throwing?

Some throttle cables get stretched. It is possible that it could only need adjustment. Depending on engine would determine if cruise uses a different cable connection.
What engine and chassis?

Odd that it runs perfect with cruise control, but not manual control.

NBeed to have moderator move to the correct forum. Maybe to class A forums.
Hasve you checked the air filter to insure it is not saturated with water?