flexing big ring

Clever Pun wrote:

I ride using the outside ring pretty much all the time... it seems as of late that the chain it tapping/catching both sides of the front dérailleur this happens less if I pull up at the same time as putting the hammer down

the crank is nice and tight.....

does anyone have any suggestions as to what the problem is and how to solve it

I take it the ring itself is straight?
There could be play in the BB, though I suspect you'd have noticed, or the chainring bolts (attaching the rings to the crank-spider) could have worked loose. Most likely, though, is that the crank is moving on the BB axle. You say it's tight, but could there be some play there, perhaps from previous damage? I think that would be consistant with the problem being less severe when you pull up, since that would help to keep the chainset in-line.

I've a horrible feeling the frame is allowing the flex.. (i.e it's cattle trucked)

That is possible, I'm afraid... What's the frame made of and how old is it?

I've had to write-off two winter-hack frames which both rusted through the chainstay where the chainring-dimple is. This manifested itself as frame flex, resulting in the chain unshipping under power. Both frames had suffered many years of abuse, even before I got them, though. I suspect, though havn't personally experienced, that a cracked weld at the BB would display similar symptoms but with rapid deterioration.

Cheers,
W.