2001 Renault Megane Will not start sometimes

HI,
My Megane 2001, 1.4 petrol has had strange tome since I bough it. First I had to take the immobiliser out as it went faulty (as mechanic said) and won't let it start. Then it ran a few hundred miles good biut strated power loss. I had to change coil pack and it went back fine. Now it has very strange problems.
1. Sometimes it won't start. Battery is ok, engine turns and revs but won;t start. So I have to push start it. Once I push start it, it just goes back normal, drives, and starts normally. This will happen again, nest day or next week, but it does happen.
2. Second problem is power loss and strange sounds from the back of car (may be exhaust) after it goes on long journeys or even does a few miles. This will make the car less accelerating going up the hills.

mechanic told that this is def a current (or crank if I misheard) problem. I just want to ask you guys if he is right? Also the power loss, do I need to change other coils pack (Prev I changed #3).

Pls help.
many thanks