I cannot see any relation between replacing power steering hoses with speedometer/tach issues ... there is nothing up front on the driver side of the engine compartment involved with the tach/speedo.
Sometimes totally unrelated items happen at the same time. My poor mechanic has to explain this often to customers.
The tach/speedo issue could be something as simple as a lose connector at the bottom of the instrument cluster. This generation of Porsche 928 has a flex circuit that connects all the items in the instrument cluster. Removal and inspection may yield the answer.
Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast