I would recommend approaching this differently. IN ( @variable ) is notoriously a pita to get to work.
Instead, use a string splitter (delimited8k, SQLCLR, whatever you like) and turn your string ('10,11,12,18,42') into a joined iTVF for the procedure. It'll take from 1 through x parameters that way and you'll avoid a lot of the headaches.
That's the incredibly short version of the task, and it'll get a lot more detailed if you're not familiar with that. Let us know if that doesn't get you on the right track and either I or someone else will spell it out when we get more time. If you need more assistance if you can post the source Proc that will help us integrate what you'll need.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
For better assistance in answering your questions[/url] | Forum Netiquette
For index/tuning help, follow these directions.[/url] |Tally Tables[/url]
Twitter: @AnyWayDBA