It is not uncommon for E30 BMWs to develop intermittent malfunction of dashboard instruments, in my case involving the coolant temperature gauge and the on board computer (OBC). The more common faults to investigate should involve the mounting stud of the temp gauge, and improving the solder surface that acts as a ground for the gauge. It is also wise the check the continuity of the traces on the board with a multimeter and resolder all the connections to the appropriate connectors.
When all this has failed to bring improvement, it may be time to try replacing the entire instrument motherboard. In my case, a new E30 M3 motherboard is hard to find. and then is expensive. The ordinary E30 motherboard is nearly identical and cheaper, except in providing a socket for the ABS warning lamp.