AMD shares fell 2 percent Monday, for a second straight day of losses as investors worry about the sustainability of the big tech stock rally this year. The graphics chip maker has now lost 6 percent of its value in just two days.



Goldman Sachs told investors to sell AMD shares Monday, questioning whether the recent cryptocurrency-driven demand for the company's graphics cards will last.



"AMD shares appreciated 13% last week on numerous media reports (CNBC, Bloomberg) highlighting a surge in GPU demand from cryptocurrency (i.e., Ethereum, Bitcoin) miners," analyst Toshiya Hari wrote in the note to clients.



"According to our checks, the RX 570 and RX 580 graphics cards are sold out at numerous retail websites including NewEgg and Microcenter. While this dynamic may provide near-term upside to fundamentals, we believe it is unlikely to become a sustainable driver of EPS," he added.



The analyst reiterated his sell rating and his $10.60 price target for AMD shares, representing 14 percent downside to Friday's close.



Hari gave three reasons for his skepticism:

1) "Rising cryptocurrency prices does not equate to sustainable GPU growth: from 2Q13 to 4Q13 the price of Bitcoin (USD/BTC) rose from $98 to $758 and the annualized run-rate of GPU unit shipments rose 4% (4Q12 vs. 4Q13). However, by 4Q14 the price of BTC had declined to $320 and the annualized run-rate of GPU shipments had declined from 61mn to 50mn."



2) "Rising cryptocurrency does not equate to sustainable ASP [average selling price] expansion: from 2Q13 to 2Q15 (i.e., the last BTC surge) AMD's implied GPU ASP declined from $37 to $25 (-32%)."



3) "Rising cryptocurrency prices does not equate to sustainable share gains: from 2Q13 to 2Q15 AMD's desktop GPU share declined from 38% to 18%."

AMD is one of the market's best-performing stocks in the past year with its shares up nearly 180 percent in the past 12 months through Monday compared with the S&P 500's 16 percent return.

Instead of AMD, Hari recommended the company's main competitor, Nvidia.



"For those looking to gain exposure to secular growth markets such as gaming, VR and data center acceleration, we would continue to recommend buying/owning NVDA (Buy)," the analyst wrote.



AMD did not immediately respond to a request for comment.



