This could be caused by several things.. First to check is the
lamp starter, if this is deffect the tube flickers erraticaly.
Worst case scenario..
The juwel aquarium hoods while aging are know to be affected by corrosion on the tube and ballast contacts. These should be inspected an cleaned with some sandpaper or a steel brush if it shows signs of corrosion. It shows a clear white/green powder like substance on teh contacts.. If corrosion gets to much it works as an insulator and prevents the power to flow correctly.. Depending on the build these hoods can have a Thermal fuse that shuts the power if it gets to hot.. These can get hot also due to corrosion on it's contacts.
I red about it quite often, that Juwel hoods are a pain in the bottoms to open for a proper inspection... I can conferm, not so long a go i DIY converted a juwel hood with 12 volt led strips myself. I wanted to take it apart and store all the parts. Storing the parts didn't work out, taking them out, cracked them beyong repair.
Just a heads up, if it aint that starter as Tim already pointed out..