NEW YORK ( TheStreet) -- CHANGE IN RATINGS Chesapeake Energy ( CHK) rated new Buy at Wunderlich. Valuation call, based on a $22 price target. Continental Resources ( CLR) rated new Hold at Wunderlich. Valuation call, based on a $72 price target. F5 Networks ( FFIV) rated new Buy at Needham. $130 price target. Stock is attractive, following a 30% selloff. Juniper Networks ( JNPR) downgraded at Needham from Buy to Hold, Needham said. Routing and enterprise switching markets are facing fundamental changes. Medtronic ( MDT) upgraded at Credit Suisse from Neutral to Outperform, Credit Suisse said. $44 price target. Emerging market should drive sales growth and the company deserves a higher multiple. Pharmacyclics ( PCYC) rated new Buy at Brean Murray. $80 price target. Lead drug continues to perform well in clinical trials. PulteGroup ( PHM) upgraded at FBR from Market Perform to Outperform, FBR Capital Markets said. $11 price target. Industry fundamentals remain strong and the company has solid operating leverage. Sina Corporation ( SINA) upgraded at Oppenheimer to Outperform from Perform, Oppenheimer said. $70 price target. Attractive valuation. Tempur-Pedic ( TPX) downgraded at Piper from Overweight to Neutral, Piper Jaffray said. $22 price target. Company could continue to struggle to grow over the next several quarters. Tempur-Pedic downgraded at William Blair from Outperform to Market Perform, William Blair said. Competitive pressures have intensified.