I'm running out of ideas. There are few more things you can try; no guarantee:
1. Raise the antenna higher
2. Try another location in the yard with a temporary mast; you might find a hot spot if you are able to monitor signal strength
3. Try another preamp like the Channel Master 7777
4. Make sure the coax shield is grounded to reject any electrical interference
If the antenna is outside, the coax shield should be grounded with a grounding block that is connected to the house electrical system ground with 10 gauge copper wire for electrical safety and to reject interference. For further compliance with the electrical code (NEC), the mast should also be grounded in a similar manner to drain any buildup of static charge, but the system will not survive a direct strike.