The 100 GPD Dow Filmtec is the worst possible choice for a membrane, it is not even a Reverse Osmosis (RO) membrane but is in fact an Nano Filter (NF) membrane. The difference is the RO is a 96-98% rejection rate or removal efficiency device intended for 3 log removal of viruses, bacteria and oocysts in drinking water while the NF is only a 90% rejection rate device intended for "Pool and Spa Use" as designated by the ANSI/NSF and is not approved for drinking water use in the USA.

Your TDS problem is the poor membrane more than anything. You will never see low TDS out of that NF membrane, it was not designed to do so.

The 90 GPD Spectrapure membranes start out life as a Dow Filmtec 75 GPD RO membrane rated at 96-98% rejection rate then Spectrapure treats them with a special proprietary process proven to increase the GPD and rejection rate then they subject them to testing, either batch testing in the case of their normal series or for $10 more you can get the individually hand tested and guaranteed in writing to be better than 98% rejection Select series membrane for only $45. A matched capillary tube flow restrictor is $5.

I use the Select series myself, my RO/DI is 5 years old and the membrane is still producing water at 99.4% rejection rate or removal efficiency today. The savings in DI has been tremendous.

When you figure that for every 2% you increase the rejection rate, you double the life of your DI resin I have save countless DI replacements over the past 5 years, actually its been 10 years as I sold my old Typhoon III at that time and bought my first MaxCap then but have since upgraded to the UHE system 5 years ago. Its money in the bank.

What is your tap water TDS, RO only TDS and final RO/DI TDS? What is your water temperature and water pressure? Do you use softened water or do you know the hardness of your water? Have you measured your exact waste ratio and if so what is it? These are all question that need to be answered to troubleshoot a RO/DI system.

Are you using Spectrapure replacement sediment and carbon block filters? Are you using Spectraure SilicaBuster DI replacement resin or cartridges? Is your DI resin fresh and if you buy in bulk is it stored in a vacuum sealed bag in the back of the refrigerator so it stays damp and out of direct lighting? Resin has a fairly short shelf life especially once it has been opened, bulk resin often is not the answer as it lloses its cationic and anionic properties when it dries out.