Fuel guage giving odd readings

I have a 110 with a sender unit without the fuel warning light. The guage reads normally when the tank is full and for about the first third of a tank, but then the guage drops to zero when braking and then comes back to normal when finished. It is not an earth issue as applying the breaks when stationary has no effect it is only when braking from speed or sharply that this happens. It was my understanding that the fuel tank was baffled to prevent a wave effect from causing this.
The tank was new a ouple of year ago is it likely one of the baffles has failed?
Or is the sender likely to be at fault which although new when the tank was fitted was an aftermarket part rather than genuine.

I've had problems with fuel level senders on my 200Tdi 90, 1st of all original sender was slowly getting more and more random so changed out for a new one. I knew it was a pattern part but when it turned up in the old blue box I was not best pleased. Anyway fitted it and it was fine for a year or so and then started behaving exactly like yours, pulled it out and the plastic ball/float was half full of derv. Ordered a bearmach replacement, fitted worked fine for about 6 months and now thats gone random as well.
When I get totally fed up with it all I will probably try swapping bits from one to another to get a good one.
If all else fails it's going to be an interview with the bank and get a genuine one.

I don't tend to really us mine I always reset the mileage and work off that as I have never trusted the gauge. But it is annoying. When it gets worse or I get some free time I will pull it out and have a look.