We had this problem with our Mk2.5 and it turned out to be the alternator which was on its way out. If you find you need a replacement alternator try Junk Yard Dog (John)......we got one for, IIRC, £40.
when belt tightened giving out 14v car then started first time over a few days while car in garage then as soon as i brought it home about 2 miles lost it again radio switched itself off wipers going slow switched off then would not start again very frustrating
Bought a Eunos last year that had sat around for a long time. On drive home car died with flat battery. On inspection toothed drive belt for alternator/ water pump had stripped all teeth. Replaced drive belt and all was well. Worth a look before you change alternator.