1990 Subaru Liberty wont start

i turn the key in the ignition and it will take a fewseconds before it connects. (that is when i start it for the first time every day). after that it works ok. but then it takes a bit longer to connect each day until eventually it wont start at all. when i try to start the car everything else is ok (like the lights etc) . i have taken it to my mechanic and he thought it was the head gasket. he removed the head and replaced 2 valves, and 1 injector was faulty. replaced injectors. after 2 months it started doing same thing. so then he thought it was battery or startermotor. so fitted new battery and second hand starter motor. alternator was ok. funny thing is that everytime i take it to the mechanic and he says its these things, then it goes ok for about 2 months and then starts with this not starting again which is doing at the moment. very frustrating as i need my car for work doing door to door sales. im losing heaps.