1999 Lexus RX 300 Car repair

The problem started on july3, car all of sudden wouldn't start. Prior to that there were no warning, so I took to auto shop and did tune up; after that it stared driving like brand new car with no problem but week later it stared again. First it wouldn't start but while it did and as I started to drive it stared to hesitated and stop working. Took back they said may be air valves, after checking that and changing the AC filter it stated driving brand new, week later same problem so went and replaced the batteries, put fuel injector cleaner in gas tank and it stared driving like brand new. But from last evening it's acting up and it started this morning but it was stop and go and flittering, hesitation. Please help