2000 cavalier

workin on a 2000 cavalierw/ 2.2 engineohv.. just put motor in ran great for a day .turn off start in morning and runs rough .took off valve cover and two rocker arms are off. i put them back on and it runs great again..the previous motor had exact same prom, but wiped out a valve..looking for advice

also motor only has 32000 miles , previous motor only had 80000 miles

Something fishy here......engine doesn't start and run "great", and the next day two rocker arms fall off....

ran car again today and no problems. still runs great,,thinking waterpump does not give enough flow and valve "grabbing" inside guide when shutoff..any ideas?
also what do u mean by "fishy"