just received the bugera 333-212 combo. I turned it on, let it warmup for a while and then started playing.

after 15minutes or so, the lights from the channels go out and the amp starts to make a strange noise that repeats every 3seconds or so. then switch it on stand-by and back on, but the noise was still there. then i turned the whole amp off and, couple of minutes later, on again and the noise was gone and I could play again. 15minutes later the same thing happend. this time i turned it on standby for a while. after a couple of minutes the lights from the channels turned back on and the amped worked again.

What is wrong with it?
The thing that puzzles me is the light going off. Otherwise, the noise would come most certainly from your domestic plug. I suggest you to take the amp to a rehearsal room provided with a power conditioner and check the amp there. If it does the same when plugged into a power conditioner (or an alimentation cable from Evidence Audio which does the same trick), then the problem is within the amp. If it works fine there, then you need a power conditioner.
Please pay attention to possible interference from radio waves, computers or TV sets, too. Valve amps are particularly sensitive to interferences. Turn everything off when playing at home or buy a radio filter and plug it into your pedals chain.
sounds like a problem with the power section

contact the seller

trying it at a different location is not a bad idea

i wouldn't turn it on unnecessarily until you can get someone qualified to look at it.

maybe a tube pin is arcing with the board. A tube may have come loose when it was in transit

maybe it is just a bad preamp tube.

if you want to start troubleshooting this yourself - swap some of the preamp tubes around and keep track of what tube is where. You can number them if you like by writing a number 1,2,3,4 on the glass with a permanent marker. Spray the pins with some contact cleaner.

otherwise - get one of these:

but seriously - I'd take it to a tech