First, I wish I had a better answer.
I would suspect the speed sensor. Due to the history you provided, the only way for it to indicate 99 mph is that it is bad. All sensors are interconnected via the PCM so an intermittent short can create all kinds of codes. Unfortunately other sensors could be corrupting the speed data. 900+ times. Somethings up. Speed sensors are usually cheap, and easy to change. In your mechanics defense these problems can be extremely difficult to fix. But $1500 buys a lot or sensors, PCMs, and wiring plugs.
If you decide to put up with it you may wish to purchase your own code reader and just check it every so often. A good Actron unit is about $150 at any parts store. It is about the size of a Palm Pilot and plugs in under the dash. It will work on all cars sold in the U.S since 1994. Plus you will sound very professional the next time you take it in for service. Good Luck.