Jumping down gears...

I'm hoping someone on here can help! I've recently built up a 531 frame with some modern components, albeit with down tube shifters. Trouble is, when I ride on even the slightest gradient or when I push off at the lights the gears shift down the block leaving me grinding along in a big gear! I've noticed even a slight tug on the cable at the downtube will shift the lever into the next sprocket, the derailleur is screwed in tight, I've tried it with two different levers with the same result. The hanger looks straight and the chain looks like it's in a straight line from sprocket through the jockey wheels, and I've experimented with different chain lengths with the same result, anyone got any other ideas?

Indexing setup properly? Cassette worn?

This is not really the thing to sort out on an internet forum. Visit your friendly LBS.

Cracked chainstay?

Too many gears?

repped

Are you using the correct size chain for the cassette/freewheel?

Are the down tube shifters friction or indexed. If solely the friction type it sounds as they are too loose. Adjust them to give greater resistance.

a slight tug on the cable at the downtube will shift the lever into the next sprocket

Definitely sounds like a friction lever with insufficent friction

a slight tug on the cable at the downtube will shift the lever into the next sprocket

Makes it sound like your shifters are indexed? If so, something's wrong with the indexing?

Actually, are you possibly just missing the piece on the left (or its equivalent)?

Yes indexed levers, I've actually tried with two different levers but both had the same problem, and secured plenty tight too!

Gear cable? Try adjusting and testing with bike in a stand.

Flex in the bottom bracket when you get out of the saddle.
This loosens the cable to the rear derailleur causing it to shift down.

See here: http://sheldonbrown.com/autoshift.html