I have the same problem. My housekeeper and alerter processes are at 100% and from what I can tell, housekeeper has been over 75% for 1h 11m 19s and the alerter 1h 1m 17s. It seems that when the housekeeper kicks in, a lot of triggers may timeout and this causes an alerts snowstorm on false alerts. Even though housekeeper now is at 100%, the zabbix server CPU is still 85-90% idle. Can housekeeper and alerter processes be parallelized such that there can be more than one of each such processes.
1)Please don't mess with StartDBSyncers and set it back to what it was before. (4 by default)
Take a look at other performance issues - poller amounts and cache usages and database performance tuning.
2)you can set Default operation step duration on zabbix web to deal with alerter process is more than 75% busy, web->configuration->Action->Operations->Default operation step duration