Yup. We shipped our share of 83MHz bus systems - not M-II 366 parts, 6x86MX-266 - and they were really, really hard to get right. You needed good RAM. decent main board (not PC Chips!), and care with your choice of video card. Even then you had to hold your tongue just right.
Why did we bother? In a word, price-performance. The 266 was just so much faster than anything else in its price range. And yes (refer my remarks in some thread or other yesterday) in those days a speed grade was still a speed grade, not one of these itty-bitty modern things.
But when the K6-266 came out, our 6x86MX-266 buying days were over. OK, the Cyrix part was marginally faster, and cheaper too, but the K6 was just so much easier to get right.
The MII 300, by the way, came later. It was no faster than the 266, just a 66MHz bus, but it was one of the easiest of all CPUs to work with, as easy as a Pentium MMX or a K6-2/450, and we sold heaps of them.
Pity you never got to know the Cyrix chips properly Mercutio. They had their own peculiar way about them, but you could have saved your customers a massive pile of money if you added all those systems up. The thing with them was not so much that they were unreliable or difficult, they were just different. In particular, you'd usually use a completely different motherboard for them. Put a 6x86 on your best-choice board for a Pentium MMX and, sure, it would work, but it wouldn't work particularly well. (I did benchmarks on this once. Very interesting.) Put it on the right main board though, and you'd blow the Intel chip right into the weeds and have enough change left over to think about more RAM or (our favourite trick this) a 7200 RPM hard drive.
And yes .... Alas, far too many of the really cheap and nasty builders used Cyrix chips. That is where so much of the Cyrix reputation came from: not the actual chip, the dreadful crap people used to ship with them, and the horrible things they used to do to the chips along the way, like running them at the wrong bus speed and multiplier, or 0.2v too high because they didn't know how to set a jumper on the mainboard.
Actually, I could divide them into three categories, the Cyrix CPUs, no, make that four:
Work perfectly but dog slow
486SLC
Horrible unreliable things unless you did exactly what they needed
486DX/2-80 (tricky bus speed) (actually, the AMD one was the worst, by the time the Cyrix ones arrived 40MHz on a 486 was not so hard)
5x86-120 (tricky bus speed)
6x86MX-233 (tricky bus speed, general weirdness)
6x86MX-266 (very tricky bus speed)
MII 366 (from your report)
Wonderful performers that only required a little specialised understanding
6x86-120 (cooling, voltage)
6x86-133 (tricky bus speed)
6x86-150
6x86-200
6x86MX-333 (three versions - some were excellent some were dreadful)
Just wonderful performers, totally fuss-free
486DLC-33
486DLC-40
486DX
5x86-100
6x86-166
6x86MX-166
6x86MX-200
6x85MX-300
