Unit works with short cycling

Question

when t/stat turns on with setting of 78deg inducer motor & blower kicks on then igintor kicks on then burner kicks on after little while burner turn-off then blower speed gets low , inducer motor still runs,green light blink with 3 times & hold again blink 3times,then inducer motor turns off, again doing the same cycle,temperature not rise more than 67deg,I am refrigeration tech,pls. give me some clue or hints to work on with,thanks