OK, so you are using the TT's built in phono preamplifier, is that right? If that's the case, the phono preamp is shorting out. This could be a cracked solder joint, dirt, loose part, or corrosion in the phono preamp. Since you're switching from phono to line to phono, which sometimes fixes the issue, that action confirms the problem is likely on the menu I've presented. If you have an electronics repair shop in your neighborhood the TT can be fixed in a few minutes.