Goldman Sachs has downgraded its rating for Admiral from 'buy' to 'neutral' ahead of the car insurance group's annual results next month after the stock's recent outperformance.The bank said that Admiral's share price suffered excessively from negative sentiment in the third quarter of 2013. "Since then, Admiral shares have risen 19%, outperforming the broader European insurance index by 16%. We now see only limited upside potential, and downgrade Admiral to 'neutral' from 'buy'."Panmure Gordon has lowered its rating for security solutions group G4S from 'hold' to 'sell', saying that the stock's current high valuation is unwarranted given its growth outlook.Analysts Mike Allen and Paul Jones said there are "too many dark clouds on the horizon" for G4S and have cut their forecasts ahead of the firm's results next month.Analysts at Goldman Sachs substituted their 'not rated' designation on shares of Vodafone with a recommendation to 'buy' due to the current undervaluation of the shares and scope for the telecommunications carrier to be taken over.Vodafone is one of the few "substantial" assets available for purchase to a rival seeking to build global scale, with the added benefit of £106bn of long-lasting agreed tax losses. Furthermore, Goldman has identified $2-7bn of potential annual synergies should Vodafone combine with a leading global operator.A whole host of analysts lowered their recommendations for Hyder Consulting on Monday after the surprise profit warning from the design and engineering consultancy group.Brokers Panmure Gordon, Investec and Numis Securities all downgraded their respective ratings on the stock after Hyder warned that full-year results are expected to be materially below current market expectations.BC