ok when you shift into gear, thats when the trans starts using vaccum, if either you have a vaccum leak or the module on the tranny is failing, it will cause a severe drop when you shift it into gear.... could be also your cam grind, you might not be putting out enough vaccum to support the added transmission at idle....or hell, timing could cause that too.... o0o o0o!!! also bad idle fuel mixture could cause it.... take a look at some of those... see what you find out