Wotcha and welcome :)
the speedo bouncing may not necessarily be the cable as it could be the wheel turning the cable is missing some teeth check the other end as well before changing the cable as it may not fix it
1.75 bar is about 25 psi which I would say is high for a standard car - I would expect about 0.8 - 1 on boost
Is it constantly sat at 1.75 or is that what it tops out at when on boost?
There is will be an over boost protection on the car which may be linked to the check engine light - that I would expect to be set at 1.2 or so for a standard car which would hold the engine back to protect it
To turn boost down you can adjust the actuator which sets what pressure the turbo will be allowed spool up to - simple device uses a spring and lever, when pressure exceeds spring tension valve opens and stops over pressurisation
hope this helps but I could do with a few more details about the tune of the car, service history, engine mods etc