I suspect you have an intermittent /corroded connection and there is nothing wrong with the alternator is the alternator belt loose? I would start by checking and cleaning all the electrical connections. On the alternator, (ALL of them) you can spray deoxit gold 100 on the disassembled terminals. if the connections have turned black, green, etc... clean all the contamination off. very often a bad connection will lead you to believe that an alternator or other part is bad, even when it's still good... even a bad connection on the computer equipment. this may send you on a wild goose chase. unplug the brainbox, relays, connectors and spray deoxit gold 100 into all that stuff, plug and unplug to distribute the contact preservative around, inside the deepest connections. going through and cleaning, checking, tightening may well solve this problem. use only HIGH QUALITY contact cleaner which is safe on PLASTIC! This is made by CAIG chemical, and it IS NOT relay cleaner. Cheap contact cleaner is inferior and may damage (melt) plastic. Relay cleaner will MELT plastic. If you have cleaned all this stuff and confirmed that there are no loose plugs, etc... your alternator can be tested for free at an auto parts store. From what you are describing, I doubt the alternator is really bad, although I will not say impossible. The deoxit spray is non shorting and does not interfere with operation when it's wet. you can spray it and run it right away, it's non flammable. Generally speaking, repair shops have no clue when it comes to automotive electrical, and you are better off finding an automotive electrical specialist. Otherwise you may wind up replacing a bunch of parts for no real verifiable reason, then the problem comes back again. Even a bad solder connection on the computer PC board could cause this. bad connections can be found by tapping, wiggling, etc.. to narrow the area where the intermittent exists.