"Every once in a while it stinks."

Sounds like the trap is drying out. When you fill it up during your clean-out activities it's OK for a while. Then it dries out again and allows sewer-gas to drift up.

The trap could be drying out if the shower is rarely used, or if it is used a lot there could be buildup in the drain line before the trap.

If the shower is rarely used, fill the trap with water, then add about 1/4 cup of mineral oil (from the drug store) to slow the evaporation of the trap water down.

If the shower is then used, the mineral oil need to be replaced.

If the bathroom is used frequently you can remove the grate from the shower floor and use a long bottle brush to clean the inside of the pipe before the trap.
Shampoo, soap, hair, and whatever you washed of yourself can build up inside the pipe and start to smell.

The trap might be getting emptied actively (instead of evaporating, which is a slower process). High winds can cause a trap to get siphoned out, but since this is a recurring thing, another possible cause is a big flush somewhere else in the house. It depends on many factors relating to the venting of your DWV system, and can be aggravated when venting becomes insufficient, due to partial blockages in an old system. Do you know the trap arm length ?

"High winds can cause a trap to get siphoned out, but since this is a recurring thing, another possible cause is a big flush somewhere else in the house."

This occurs when the vent system is incorrectly designed or (as noted) damaged by clogs.

The OP has not bothered to supply any more information.

Maybe they solved the problem.

Thanks Guys. Just getting back so sorry for the delay. This shower gets used every day so I'm sure it's not drying out. I don't know how deep the trap is but I have a large, wide, toilet bowl brush I'm going to run down there. I'll let you know how it works when I'm done.

Do they make flexible bottle brushes wide enough for this purpose? Maybe at Home Depot? Thanks again.

Jeff