CEL on hills/highway

My CEL keeps coming on the exactly same spot to work every morning, accelerating up a hill in 3rd/4th gear.
It also comes on sometimes in higher speed zones/longer trips. What might be causing it?
 
For reference the car seems to be operating fine. I can't check for codes as it brings up no errors as the CEL goes away when I turn the motor off (ignition still on).

just_ace
To check for error codes you need to bridge E1 and TE1 in the diagnostics box.

I know, but Can you do this with the motor still running?

The ECU will store the error codes, so you can turn it off, bridge the terminals & retrieve the code.
 
You're not running a Power FC though are you?

Code is 42 (No Speed signal) This happens when no SPD signal has been received for more than 8 seconds with the car between 2 and 5k RPM (hence it happening in the same spot or highway driving).
 
I'm in progress of changing the speedo nylon gear so i'll clear the code once I replace it.
Good that it wasn't an actual engine issue. :)