Ashland ( ASH) downgraded at Credit Suisse to Underperform from Neutral. Sees earnings pressure continuing after Q4 pre-announcement. Maintained $66 target price. Clearwire ( CLWR) upgraded at Jefferies to Buy rating from Hold. Price target jumps to $34 from $24 and FY 2008 EPS estimates dip to ($4.88) from ($4.77). Citrix Systems ( CTXS) upgraded at Deutsche Bank to a Buy rating on increasing conviction that XenSoure is a legitimate #2 vendor in the virtualization market. Marathon Oil ( MRO) upgraded at Goldman Sachs to a Buy rating from NR (Not Rated) based on view that company is most favorably leveraged refining pick among integrated oil plays. Note current seasonal weakness in margins as buying opportunity. Price target at $72. Nelnet ( NNI) downgraded at FBR to an Underperform rating as growth challenges outweigh liquidation value. Occidental Petroleum ( OXY) downgraded at Goldman Sachs to Neutral from Buy due to valuation. Note 44% gain since January versus 4% increase in S&P 500. Maintain favorable view of company fundamentals, but see better upside opportunity in MRO. Price target raised to $73 from $69. Radiation Therapy ( RTSX) downgraded at Deutsche Bank to a Hold rating on announcement of LBO. IMS Health ( RX) upgraded at FBR to an Outperform rating based on significant pullback in shares which has taken valuation to all-time lows. Target price is now $29.