Hey Mick... good to catch you the other day!
They can certainly cause problems if the shape is particularly irregular... especially if the nugget is viewed by the coil "edge on" or has a hole in it. Hence one should always re-scan a target with the coil at a 90 degree angle to the angle you first approached the target from. Be sure to scrape away the topsoil from the surface with your foot or pick before rescanning because the minerals deposited on the topsoil tend to hamper the signal. Most nuggets tend to be softer in tone and fainter. People sometimes describe this as a "Mellow Sound". However, a nugget near the surface or on bedrock near the coil will sound sharper and louder. A suitable coil (DD HF Coil) and a good Ground Balance is a must. Be sure to turn off your Ground Tracking when you come across a target or your detector might eventually ignore the target before you get to digging it.
If the TID numbers bounce around a little, that's not too unusual although a solid lock is preferred with a number on either side flickering being okay. Remember that anything at depth will confuse the Discrimination of any detector... including the X-Terra and even the GPX's. If the numbers vary and are out of sequence, there's a chance the target may be junk... especially if there's some minus numbers there. But if the numbers flash and are in sequence, the odds are good that you've found a legitimate target worth investigating. A lot of iron tends to bounce around in non-sequential numbers like -12 +10 -4 etc whereas a lot of good targets tend to bounce sequentially like this: +4 +6 +10 (sometimes the numbers flicker in reverse but if they are close to being sequential, I tend to dig them).
Anything like a -12 is likely to be iron. Some members have found Gold nuggets that started out with a -4 but became a +10 when closer to the coil after digging.
Cheers!
Marco
