The standard tubes (T12) and slim tubes (T8) are NOT compatible and not meant to work in the same fixtures. They require a different ballast.

The T8 tube went bad quickly because the T12 ballast was forcing the wrong amount of current through it.

Your light now flashes because the T8 tube damaged the ballast, which is now overheating and cycling its thermal protector.

You now need a new ballast, and strong encouragement to read the warnings that say things like "USE ONLY F40T12 LAMPS" or something of the sort.

Thanks for your quick reply pharkus. I am a novice when it comes to electrical. How complicated it is now to change the ballast? Do I need to hire someone to do it or is it something that I can take care off?

It may actually be cheaper to replace the fixture than just the ballast.

The newer electronic ballasts are much quieter than the old transformer ballasts.

Ballasts are not hard to replace, you will likely need a few small wire nuts since they come with stub wires.

Thanks brickeyee for your reply. Will try to replace the ballast as I already have bought new bulbs.

Since you are replacing the ballast you can convert the fiture to the newer T8s. Simply buy a T8 ballast instead of the T12 Version. Soon you won't be able to buy the T12 anyway. They are being phased out.