If your solar regulator
voltage setting is higher than your alternator regulator
voltage setting, then your alternator regulator would turn off the alternator at high engine speeds since the solar regulator would be charging the batteries
to that higher voltage. If your tach signal coming from the engine is mis-wired at the alternator or at your keyswitch, the shutting down of the alternator may also interrupt the tach signal.
I would think this would cause the tach to go to zero rather than be stuck at a lower value.
I wonder does the engine sound like it is at the higher rpm even if the tach does not reflect it?