SylvanLMP
Hi Chris,
You have to love so called plumbers willing to work for contractors who normally look to get the lowest possible bid and thus do not normally use the better materials.
What your describing is a classic with a pressure balancing valve rather then a temperature type.
Best bet is to send me a private E mail and give me the name of the shower body you have and I maybe able to guide you in how to repair it YOURSELF properly.
Either that or get all your neighbors contact the builders plumber and tell him HE HAS A PROBLEM.
99.98% of the legitimate Licensed Master plumbers I know do stand by their installations even if it is specified by a home builder looking for bottom line materials
Remember hiring the actual Master plumber to work on the job is for your protection.
Ask to see what formal training the mechanic has and do ask for an insurance certificate.
This is for your protection GOOD LUCK 
