First, the problem you describe-- FSE 0911-- is very common among older
IIgs's. It is a malfunctioning ADB controller IC. A partial 'cure' was
described in Computist some years ago. (Basically, you stabilize IC temp
by taping a small 12V bulb to it and use the 12V fan line to power the
bulb. This works pretty well.)
However, a true fix usually consists of swapping motherboards. It is
difficult to find anyone who has the part and is willing to remove and
solder-in a new IC. (AND, what's worse, no real guarantee that the 'new
IC' will be any better!)
Rubywand