A few things;
1. Double check connections at receiver, in attic and at lnb (in front of dish).
2. The dish could be slightly out of position - however, if you are getting very strong signal (check percentage and quality -as should be displayed via your receiver) - then absolutely nothing - then more likely a loose connection - unless this is happening when theres strong winds, you lose signal completely when wind picks up and get signal back when theres a lull?? Is the dish in an exposed location? What size is it?
3. Could be due to water getting into the cable - check cable and see if theres any cuts in it. This can happen if theres any slack and it keeps scratching off something. eg. if drapped over slates/tiles on roof - and not secured - eventually, cable will be damaged and water will get in. As well as degrading signal, this could potentially kill you set top box as well. Capillary action within cable will draw water to other end of cable run.
Check all of the above - if no success, post back here.
Where are you located?