Can't keep site up for Google

There has to be something wrong! I got my site on Bing and Yahoo in good working order, but for some reason on Google we come and go. Example: Today our one page describing a product we make if searched is listed by Google position 1 or 2. Our competitors are listed below us so no problem, right! After about five or six days the same search reveals Our competitor is now in position 1 folowed by other simular companies and we are completely gone even looked beyond 100 positions and still not there. I could understand dropping a few positions even 10 or 20 in worse cases but to drop completely off the web !! I have checked Google Annalytics to death and according to Google webmaster tools there is no problems,warnings,or malware…our site is clean. We are gone from the web for about three or four weeks then we will show up for another 5 days and then gone again. If you google our home page we show up great but in our business we need our product pages to get hits. This has been going on for about a year. We have consulted with free SEO guroos and now are being referred to SEO company that for a pretty good fee says they will find the problem with ofcourse a huge contract. We are a small company, this fee would cover at least three salleries. Please!!! is anyone seen this before and can help us !!!