Call CC and get them to look at it, as this does not sound right, failing that...
When they upgrade a 125 to a 150, they remove the rubber pipe going from the inlet manifold to the MAP unit (the small rectangular box next to the ecu). This leaves the input of the MAP unit open to the atmosphere, but they change the map to suite this configuration.
Things to check.
The 150 has a pair of throttle bodies linked in the middle. If these are not "balanced", then you can get similar symptoms.
Under your accelerator peddle there should be a bolt acting as a stop. If this is too low and you you have a heavy right foot, the throttle cable can put a strain on the linage at the throttle body end causing it to go out of adjustment, I believe.
The econoseal connector on the Lambda probe, have you gone over any harsh speed bumps recently. Check that the connector is tight.
Check the plug leads are on tight.
You have to undo three bolts + a plastic clip that retains the accelerator cable in order to remove the plastic cover over the plug leads. Be careful when you replace the bolts, as I think some were supplied too long, and when you tighten them, they pull the brass nut out of the camshaft cover. SHould have one or two washers under them to stop this happening. It might just be one lead that is loose. ps dont check with the engine running
Has your battery got a date label on it? If so what is the date?
Have you checked the acid level in the battery?
Are the sides of the battery bulging?
Hope this helps, otherwise Blatmail me as I also have a 125 that has been upgraded to a 150, and we can compare notes.
Edited by - Steve Cornford on 10 Jun 2011 23:05:07