Low hot water pressure at bath / shower?

I have a basement water heater. Kitchen, laundry and 1/2 bath on ground floor. 2 sinks and shower on 2nd floor.

Hot water pressure is strong everywhere in the house except at shower / tub.

I can run both sinks up stairs full hot and get nearly 2gpm simultaneously, but I only get around 1.0 at the bath.

The bath / shower valve is an old ball-type delta. I have changed the ball and seals which made no improvement.

I'm left thinking there is a blockage of some kind between the sink and the shower valve. The valve itself seems most likely. I'm not a plumber, but I'm definitely a DIY'er. I just don't want to tear into this (which means cutting into adjacent bedroom wall) unless I've exhausted any other possibility.

John, the flow rates I mentioned were measured, not eyeballed. Shower used to have great pressure. It's a dual head set up. Both had plenty of pressure. Now, using two is a waste b/c the water just trickles out. Using just one gives adequate pressure.

I already tried blasting it w/the stem removed (one handle - ball type control mechanism - push up to open, left hot, right cold.). Coldwater blasted. Hot water, as expected, came out much slower. I couldn't even tell the hot water was flowing. I'm sure it was, but the cold was blasting out w/so much force I couldn't tell what was hot. I've read several tutorial on back flushing. I'm going to try that next.

I waited it out.

A couple weeks later I noticed intermittent good pressure. Strong for a few seconds, then weak, then a flash of strong, then weak etc.

Then I noticed a few small pieces of plastic (like from a heavy duty trash bag) in the tub. Pea sized or smaller. Over the next day or two a couple square inches of the stuff had come out. I now have strong pressure.

So thanks