how can i fix my ford fiesta 1.1 93-94 model?

i have a ford fiesta M reg which starts sometimes but not always! i have had the AA out and it started first time!!! then the next day it wouldnt. he checked the crankshaft sensor, which he cleaned. that was ok. then he checked the fuel pump and fuel going into the fuel injector, that was ok. still no start. he then took the air filter box completely off and the car started! i have since had the filters all checked and cleaned but still it is tempermental! i tried starting it last night by taking the filter box off and it would not start and yet first thing this morning it started from just turning the key. and advice would be grateful