Forget halo. Your detector, which is a good one, should be doing far better. Though your doing better in air tests I should still check out the state of the battery terminals and give a good clean if required.
Only other things it could be is incorrect ground balance, a fault in the detector itself or perhaps the discrimination knob has been off and replaced incorrectly so your using a much higher level of discrimination than the pre set position indicates (the pre set is a little high anyhow unless your just working rubbish infested parks). Also see if TR mode (that doesn't use G.B.) is any better than the other modes though you will have to be on sand or mild soil.
Last resort is to try another coil just in case but it doesn't really sound like a coil fault. Best of luck.