This has happened to me in the past, and I am still not quite sure what causes it, but it started happening to me again and it really bugs me. I could be walking anywhere (flat map, stairs, etc), and all of a sudden I will stop walking. I start walking again when I press the forward key, but it's annoying to have to keep pressing the forward key over and over again.
Okay, this is definitely a necro, but I have this exact problem. I'm running Windows XP, the keyboard works just fine, I can move left right down jump etc but walking forward randomly stops.
I have the same problem fairly often. In addition to stopping for no apparent reason, frequently when it starts moving again I'll find myself suddenly facing any direction other than the way I was going when forward movement stopped. This has killed me about a dozen times in just the last week or so. ... walking past a lava lake with very little clearance and suddenly freeze, next thing I know I'm at the bottom of the lava lake because when I started moving again it was in the worst possible direction.
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