Frequent red eyes for 1 week every 1 mon

I have redness problem in my eyes. every 1month (almost) one of my eye start red at one corner and becomes red entire eye . during this 1 week 4 days are unbearable pain and it will will be normal at end . i have this problem almost form past 10 years and i have visited few hospitals they did most diagnosis tests , all are normal ...so they are thinking its bc of  alergy might be dust/weather... but not sure about exact reason and everytime they assured like no problem with eyes. i have used all medicines and eyedrops . execpt pred forte nothing helpfull. i am using pred forte from long , its unable to prevent redeye . but it decreasing the time from 1 week to 5 days . please help me if you have seen mycase before . and let me know if you need more reports on this. Note: red eyes frequency 1 week to 5 weeks and the duration is from 4 days to 10 days