95 Bonneville transmission acting up

[390Merc]

95 Bonneville transmission issues! Had my trans rebuilt about 9 months ago cause the reaction drum failed finally at way over 200k miles. Sometimes since then it seems to take a little longer than normal for reverse to engage after shifting into reverse, but never really an issue.
In the last month or so, it will occasionally not go into reverse at all. If I'm someplace where I can pull forward a short distance, then it may or may not go into reverse just fine. If you rev it up to 1500-2000 rpm you can hear a slight clunk at a steady interval, like something catching but not quite doing its job.
Last week I went to make a pass and as the trans kicked down out of OD, a very loud bang was heard and felt, like hitting a large rock. I backed out of the throttle and drove as everyone else was after that.
Just in the past week the downshift from second to first gear is becoming very harsh. At first it was every now and then, and since yesterday it is every time now. I thought I could outsmart it and shift into neutral before it makes the 2-1 downshift and simply drop it back into gear when stopped, but that resulted in a very harsh clunk, like a centrifugal clutch catching after slightly accelerating. Once I was afraid it was not going to go back into drive.
Its 2 weeks until the only trans shop I trust will be able to look at it.
Thinking of dumping a can of sea foam trans stuff in it to see if it changes anything until then. I don't want to drive it any more as it is.
The trans guy says he'll have to pull the sidecover off and check it out, probably a band or two needing replaced at least.

[Mbysoon?]

I'm no expert, but I've torn into my fair share of 4L60E's from Camaro SS's. Usually when someone describes something like that the valve body needs to be replaced. With 200k miles I would recommend pulling a used trans and rebuilding it yourself. It's not as difficult as you think it is. There might be a good DIY out there that you can dig up. Don't dump seafoam in it lol.

[390Merc]

I drove it about a 150 mile round trip yesterday because it was mostly highway and minimal possibilities of having to stop and start. I found that if I downshift it manually, then when stopped, shift back up to OD, it only makes a little clunk between one of the gears when upshifting while stopped. Seems to be in between 2nd & 3rd.
Trans was rebuilt by a very reputable individual here in my area last year. I asked him last week if it sounded like a TCC pressure solenoid issue and he said no, not with what its doing.

[Mbysoon?]

Then it sounds even more like the valve body, when you downshift manually it forces more pressure so that's why you're feeling a less violent downshift. I would think if it was a bad it would slip instead of clunk really violently.

[390Merc]

No slipping. Before I figured out I could downshift it I tried putting it in Neutral before it would have a chance to downshift into low gear on its own. Then while stopped, or slowly rolling, it seemed like it stayed in Neutral after shifting back into gear until I revved it up just a little, then again, it would clunk kind of hard into gear. Trans guy will be able to check it out in a couple weeks so I'm just driving other cars mostly until then. Don't want to hurt it by driving where I have to start and stop very much.