NEW GUY WITH QUESTIONS

Good morning all.
I am new to this forum and really need some technical advice on my W203 722.9 gearbox.
After a while of driving I notice that it feels like it is losing drive. I can then shift manual to any other gear and nothing happens.
I have to pull off the road then and when I switch to park and then to drive you feel a hard bang from the gearbox.
At this stage I switch off. Remove key. Wait a few seconds.
Start and there it goes normally like nothing happened.
I bought the car second hand so I do not know when last the gearbox was serviced.
Car is also long out of warranty.
I am a DIY guy but really need some advice on this.







