The 3 speed in my 260C shifts around 3000rpm from first to second even if my foot is fully down on the loud pedal. I understand it is a jatco 3N71B which is almost completely controlled by engine vacuum? I have checked the vacuum line going into the servo and it's all clear and working fine. Anything else to look for? It shifts well from second to third and will happily rev out in second to redline.

If I hold it in first gear using the shifter it will rev right out but when in "D" it won't go beyond 3000ish before changing up. I put some fresh castrol transmax Z in it but no change.

Any ideas?

There's a button switch on the accelerator mechanisim. Check under the dash on the left side of the steering to see if it's broken or has been unplugged, this somtimes happens during stereo installations.