XM (XMSR) shares dip as the satellite radio shop sees another slowdown in subscriber growth.The Washington, D.C., pay radio broadcaster managed to hold third quarter losses to $145 million, or 47 cents a share, roughly flat with the second quarter performance, but down from the year-ago loss of $85 million, or 32 cents a share. Sales were $287 million, up from $277 million in the prior quarter. Analysts were looking for an adjusted net loss of 44 cents on revenue of $287 million, according to Yahoo Finance. XM added 310,000 net new subscribers in the third quarter for a total of 8.5 million users. The pace of user growth, however, continued to slow. In the second quarter, for example, XM added 338,000 subscribers. Last year, XM added 286,000 net new subscribers in the third quarter. It is these runaway costs and steadily slowing user growth that forced XM and sole rival Sirius ( SIRI) to propose a merger earlier this year. Though regulators are still reviewing the combination, key areas like the average cost to win a customer have been stubbornly high. The so-called subscriber acquisition cost for XM in the third quarter was $70, up from $59 in the year-ago period. XM did have some solid improvements in other areas, though. The monthly customer defection rate fell to 1.67% from churn of 1.82%. And the average revenue per user remained steady at about $10.17, up 2 cents from last year. XM shares fell 9 cents to $15.50 and Sirius was unchanged at $3.63 in early trading Thursday.