Pressure transmitter reading momentarily off in WinCC PC

Hello arocon;Before you check on the WinCC side, I would look at the status of the analog signal in the S7-400.
Usually there is a scaling block used to set the engineering units on an anlog value from the field. Trace the PIWxxx value for the transmitter, see if the value is stable (try a VAT table for example), then follow it to the scaling block. Check whether the signal is stable at the entrance and the exit of the blocks.
If the signal is stable in the PLC, I would look in the Tag Management (in WinCC) for this transmitter value. If you are online in Tag Management, and you go down to the transmitter tag, placing the mouse cursor on the tag name will bring up a tooltip with status data on the connection. A "bad" conection will indicate communication problems between the PLC and the WinCC, possibly in the driver.
Come bac to us when you have more details.
Hope this helps,
Daniel Chartier

"Momentarily off" how? Incorrect value?

Are you scaling the value in the PLC? Are you assigning a value in the plc to it's register once ? Using multiple assigns to a variable in a PLC when scaling is sort of common, but HMI's don't scan synchronously to the PLC, and can occasionally get one of the intermediate values.