Tub Toilet Drain Issue

Standard 5X8 Bathroom configuration. Tub, Toilet, and Sink on the same wall.

Vent goes throught the attic.

Toilet and sink work fine.

When showering normal drain for first 3-4 minutes, then toilet starts gurgling then bowl empties. Shower continues to drain normally. Flush toilet and it continues normal operation

Any Clues??

It could be a vent clog or a minor drain clog. Have your vent and/or toilet drain cabled