another possible cause of the stiffness you describe could be a worn accelerator cable. I had this in a previous car (a mini..), the cable started to deteriorate inside the sheath and as the wire strands started to break they rubbed and caught on the outer sheath. Felt stiffer at first, then started to catch, and eventually as I realised what it might be, it broke, leaving me stranded. I hope yours is just in need of a bit of 'lube. I don't know how much of a pain in the ass changing tha acc' cable in a 155 is.
The pedal should feel stiffer than in some of the cars that have 'feather touch' pedals, but if you're really having to work to use it, there's definately something wrong somewhere.
If 'lube, and then replacement of the cable doesn't help, then there must be something in the throttle housing fouling the mechanism - but I would be very surprised if that was the case.