Try it with sonication.

your ethanol maybe is not pure you can try with ultrafiltation and vacuum distillation after this for me is extraction  with abd solutions

Few suggestions-

1. DMSO should not be used at 5% final concentration as they would be toxic to cells.

2. Repeated dilutions of your phenolic extract (and then measure your total phenolic concentrations by Folin Ciocalteau's method) would help you use a range in say nano-grams or micrograms of total extracts to the cells (to get a acceptable bioactivity!). If you are using at mg levels "phenolics have a property to bind to most proteins (those present in FBS and your media components and those proteins released by cells to extracellular media)  non-specifically and make them precipitate nicely" (which you see after centrifugation).

3. Serial dilute your stock methanolic extract in culture media itself to achieve more homogeneity in treatment.

4. Phenolics do form colors (oxidation etc. and polyphenolics, flavonoids in extracts) and can nicely interfere in your Mossman/ viability detection methods using OD/ colorimetry. So in control or blank use the same amount of phenolic extracts in cell culture media to be sure of cell-viability (that it is from the compounds and not false readings from a spectrophotometer due to compounds). Cloudiness (precipitation of proteins as phenolic-protein complexes due to very high phenolic concentration) also can interfere in determining a false viability. 

5. If initially your extracts are not getting solubilized (instead of using force- sonication!) you can use larger volumes of DMSO or water with (a Nonidet (NP) 40 : non-ionic detergents up to say 5-20 %) to solubilize your extracts and hence use in your cell culture experiments.

hope these help. best wishes.

Many thanks for all the sugestions.