In the past few days, "adaptive" WUs by Pablo were downloaded by this host; so I tried to push up the GPU clock, which was no problem. It ran at 1300MHz, all the time.
Yesterday evening, when a BNBS was downloaded, I turned the clock back to 1200MHz (as I did before with BNBS WUs, in order to have the GPU run stable at 1200MHz) - and this morning I saw that again it had automatically switched to "base clock" 1058MHz. Now it obviously doesn't even run stable at 1200MHz any more, with BNBS.
So this was the final proof that this GPU has problems with the full load imposed on by BNBS tasks.

strange thins here: although the "Edit" button was still shown with my previous posting, after adding some text I got the meassage that I cannot edit the postin any longer.

Okay, so the text comes here:

after having run for about 1 hour, right now the GPU once more switches back to 1058MHz (TDP around 60%, according to NVIDIA Inspector plus GPU-Z).
Any idea how I could increase the clock without always having to reboot the whole system for this purpose?
What I tried already was to terminate and restart BOINC - however, this did not help.

When this morning I took the first look at my PCs, I noticed the following situation on both the one with the GTX970 and also the one with the GTX750ti (both crunching a PABLO_adaptive_goal_KIX):

In the NVIDIA Inspector, the GPU clock was down at 540 MHz(!), Memory clock 2700MHz (default), GPU Load 0, Power between 84 and 89%. Changing the clock values by the sliders not possible.

In GPU-Z, no values were shown at all for GPU clock, memory clock, GPU load, Video Engine load (which normally is 0 anyway) - no values means a "-" in the fields were normally values (or "0") are shown. Power consumption shows same values as the Inspector.

However, the "progress" column of the BOINC manager shows a progress in the percentage; as it seems to me (but I might be mistaken) with about same speed as usual.

What's going on with these two cards?

Meanwhile, the WU got finished, and I received credit.
Looking up the Stderr shows the following:

...
<stderr_txt>
# GPU [GeForce GTX 750 Ti] Platform [Windows] Rev [3212] VERSION [65]
# SWAN Device 0 :
# Name : GeForce GTX 750 Ti
# ECC : Disabled
# Global mem : 2048MB
# Capability : 5.0
# PCI ID : 0000:01:00.0
# Device clock : 1137MHz
# Memory clock : 2700MHz
# Memory width : 128bit
# Driver version : r370_00 : 37290
# GPU 0 : 55C
# GPU 0 : 56C
# GPU 0 : 57C
# GPU 0 : 58C
# GPU 0 : 59C
# GPU 0 : 60C
# GPU 0 : 61C
# GPU 0 : 62C
SWAN : FATAL : Cuda driver error 999 in file 'swanlibnv2.cpp' in line 1965.
# SWAN swan_assert 0
...

any idea what this exactly means?

This phenomen that when crunching certain WUs, the GPU automatically switches the clock rate back to default 1058MHz is really annoying by now.
Currently, the GPU crunches

e1s5_ubiquitin_100ns_4-ADRIA_FOLDGREED50_crystal_ss_contacts_100_ubiquitin_4-0-2-RND5069_0

which seems to put quite a outragious strain on the card; it's been running for 32 hours now and has reached 60% progress only.
Whenever I try to set the clock to even a little above 1058MHz, it switches back to 1058MHz within a minute.
Power usage oscillates between 58 and 75%.

Since installation of crunching software acemd_918-80 (plus the newer NVIDIA driver 381.65) the situation has become even worse.

Not only that beyond certain (mild) GPU overclocking the clock, after some unknown falls back to "default clock" 1058MHZ - now crunching comes to a halt completely. So if the PC is unattended for a while and I don't notice this right away, there is an inadvertant pause in crunching for several hours, or even up to a day.

As I said before, this does NOT happen with the GTX750Ti which runs in a Windows XP system (with crunching software 849-65 and driver 368.81).
When, two weeks ago, it looked like that XP would no longer be supported by GPUGRID, I installed Windows 10 on a separate partition and ran GPUGRID there, with acemd_918-80 and driver 381.65; and - no surprise - the same problem as on the other PC.
Meanwhile, since there exists a software for XP again, I changed back to XP, and this runs without problems.

So now, at least, it'c clear for me that nothing is wroing with the video card; it's the crunching software and/or the driver which makes these funny things happen (this morning, I had this on the PC with the GTX970, too).

About a week ago, someone else wrote in another thread here in the forum that same thing happens with his card.

Really too bad :-(