In addition to what ricin said to read, also try www.fixyourownprinter.com . My old laserjet was having trouble, and their repair kit was inexpensive and included an amazingly complete instruction video.

Mine was a laserjet 3p, not a 3si, but my basic understanding is that these old laserjets have certain things that wear out rather predictably at X number of years, so it's not surprising that your laserjet is giving you grief right now since mine is approximately the same age.