Complex sentence: On lap 137, there came another series of green-flag pit stops, with Stewart retaining the lead, before the fifth caution came out on lap 144 and the sixth one on lap 155.
Split into simpler sentences: On lap 137, there came another round of green-flag pit stops, with Stewart still taking the lead. The 5th caution came out on lap 144 and the sixth one came on lap 155.