Car not starting

I have a Golf 2 1.8 16v KR engine which has a problem with starting. Firstly the problem started after I found my security system being tampered with after being away on duties for two weeks, I found door locks open and my alarm sensor in the car removed upon my return. The car started fine at first and drove it two days, then it started jerking whilst driving and I suspected fuel filter which I replaced. The car battled to start but eventually started and ran only for three minutes and it cut off. I checked all the fuses which were okay and the fuel pump which was priming. I gave up and left the car for about a month as I was using my second car. I decided to tackle it again and it would not start, then decided to take it to auto electrician to remove the whole alarm system as a first test as I can always fit a new one if it is not a problem. The immobilizer was bypassed instead and the car started amid difficulty. They diagnosed my battery as being faulty even though it was brand new. I returned it to where I bought it and was given a battery to use for time being which is small compared to my one. I got the car started after few turns and it ran until the car cut out after going over a speed hump. I could not get it started until I got it jump started after the battery seemed to have been running down. I drove to my place, switched it off, started again, battled but started, switched it off again but now it would not start. I do not want to crank it anymore as I would drain the battery again, but why is it cranking that long now?

The vehicle was running fine with no problems before my security system was tampered with or could it be something else?