Proposal Vs. Contract

pa remodeler

Hello,
I work for a design/build company in central pa. We have been having some trouble setting up a process for obtaining bids that are accurate for doing a contract. Currently we have 5 drawing phases. Schematic, design development, final design, contract drawings, and working drawings.
Our current process has us going up to the final design phase with a "guestimate" on how much the project will cost. Once we get to final design we meet with subcontractors on site so they can give us a proposal. This gives us hard numbers to give the client a preliminary estimate.
After this phase is complete there may be some minor changes, but for the most part the design doesn't change. At what point should you meet with the subs again to re-estimate the job to give us accurate numbers? Is there any easier way to handle this?
We have been runing into problems where the sub has plans form the final desing stage that don't 100% match what is on the working drawings. Then they are requiring a change order for what has changed.
Does anyone have a good process for keeping bids consistant with the drawings?

Thanks in advance.
Cory