I had a similar problem when i picked up my cosworth.I bought it from a dealer who had got the yts boy to clean the engine with a jet wash.In the end i found that water was in the ignition amplifier terminal on the inner wing.I blew it out and wd40'd it.It has been fine ever since.If this is the case with yours then check all the connectors for moisture and wd40 them anyway.Hope this helps.