Not trying to teach you to suck eggs, but mine was doing a similar thing, intermittently. I tried cleaning the contacts with a bit of emery, no good. Replaced the fuse with brand new ones (rather than ones I had lying around, still no good. I found if I briefly (not long enough to cause a problem) bridged the fuse contacts with a wire lead with alligator clips, all worked fine. The fix ended up being (disconnect battery first) and cleaned the contacts THOROUGHLY using a very fine round file (like the ones for sharpening chainsaws) to clean all the crud off the contacts, spray with CRC (electrical lubricant/protectant) to stop further corrosion...