It's a high traffic website, but I guess high load is not because of traffic because many times it runs smooth on same traffic. I researched a lot and optimized Apache, switched off logging modules to save resources and done whatever may be cause of load. Then finally I installed devel on site and check using "Collect query info", "Display query log" and I found that this particular query taking too much time (more than 2000 ms)
UPDATE cache SET data = 'a:2:{s:7:\"profile\";a:36545:{s:32:\"c21f969b5f03d33d43e04f8f136e7682\";a:17:{s:6:\"layout\";s:7:\"default\";s:14:\"layout_desktop\";s:7:\"default\";s:15:\"layout_handheld\";s:2:\"-1\";s:13:\"layout_iphone\";s:2:\"-1\";s:4:\"menu\";s:13:\"primary-links\";s:10:\"menu_style\";s:4:\"mega\";s:11:\"option_font\";s:1:\"0\";s:14:\"option_layouts\";s:1:\"0\";s:11:\"option_menu\";s:1:\"0\";s:13:\"option_screen\";s:1:\"1\";s:13:\"option_t3logo\";s:1:\"1\";s..........................................................
and page getting loaded and loaded to infinite time, it never completes. I am sure this is the only reason for high load on server.
APC is already installed on my seerver.