Electrical problems can be very intermittant, and very hard to find if it is a component that has it's "internal organs" failing.
As to it just starting to act up one day, it probably means that a connection that was at the failing point, did.
If it's in the budget, change the HFM unit, and see if it fixes it. BTW, have you looked into alternate parts sources, like this site?
1998 C230 "Black Betty"