searching for posts with similar tags

hi
i am trying to find similar records, ie. look up Posts with the same
tag, kinda like if u like this then you may be interested in this
also...

but am having a strange problem.

if i post a tag name with the url and use this code
tag_name = params[:id]
@posts=Tag.find_by_name(tag_name).tagged

it all works. but if i do this

@randomtag = @posts.tags.find(:first, :order => 'RAND()')
@suggestion = Tag.find_by_name(@randomtag).tagged

it breaks. i can see that @randomtag is returning the corrrect result.
and also if i put
@suggestion = Tag.find_by_name('books').tagged

then it works. [sound of me scratching my head]

please help.