I am ordering my third cartridge in 10 months from Pfister for a "Rancho" model single handle shower valve. Contractor installed - cold side will get down to a literal trickle, but you hit the cutover point turning up the single handed lever, and hot has full pressure. Cant imagine the cartridges keep failing but contractor says thats what it is. Any ideas? We are on well water, but no other faucet, aerator or anything else has had to be addressed. Only cold side shower (single handle valve), and this is cartridge 3. Lived in last house 11 years on well water and never replaced a cartridge. Any input is appreciated.