I have a Yamaha 250 special 1989 which has been completely reliable up to this year when the following intermittent problem started. I start the bike - no problem - drive about 400 metres and then suddenly it backfires loudly and dies. When I try to restart it it refuses for about three minutes by which time battery begins to fade. Then suddenly for no logical reason it starts up again (usually after I pushstart it) and gives no more trouble until the next day.......HELP!!!!