We need to know what the bottom bracket brand is - You can tell it is a crank problem because the click always happens at the same time, regardless of the gear that you are in. That's what's happening, right? There are several things that could cause a click, up to and including a crapped out weld in the frame...a seat rail broken or not quite bedded in the clamp (eliminate the seatpost, etc, by riding out of the saddle)...a ball broken in the bearings...a brinneled axle or cup (or both...a ball falls into the dimple with a click)...a crankbolt not tight, causing the crankarm to rock on the spindle...the chainring bolts not quite tight...bum bearings in the pedals...a cage that is a little loose on the pedals...I suppose you could get a click from a cracked crankarm, too, just not for very long. Oh, and the wrong sized bearings (like super record cups with nuovo record bearings)...

Describe the whole setup.