[Rick-sta]

I've just changed the 2/4 duty solenoid on my dad's 75 auto as it had failed. Done the resistance tests with a multimeter and the reading for all the other solenoids was fine but the 2/4 duty solenoid showed no reading at all.

The symptoms was very hard gear changes like as if it was banging into each gear. Even when stationary and switching to D or R it was banging into the gear and jolting the car. It still went into all gears, but very harshly.

To me your issue sounds like a possible maf issue. Try disconnecting the maf. When the maf needed replacing on my dad's 75 a few years back the car really struggled to rev in gears. Would get to about 2k and really struggle to rev higher when driving, so was also refusing to change up gears. Was surprised as usually the maf reduces power at low revs below about 2k when needing replacing. Could be worth a try just unplugging the maf.