The bug
This is a well known bug, but I only saw one report for it, which did not have a good description. Under certain conditions, when you start sprinting, the FOV will repeatedly switch between the normal and sprinting levels. Your speed stays at the normal walking pace, and the particles appear in bursts, while the FOV is in the sprinting position.
I know several different events trigger it, but I have found one way to faithfully recreate it after 1-3 tries. Simply hold shift for a moment, then let go, and double tap W the instant you let go. It occurs almost every time. I've also recreated it a couple times by sprinting over a half slab at various positions. The sneaking method no longer seems to work, and there currently is no known way to consistently reproduce this bug.
This bug is especially annoying for me on parkour maps, since I tend to hold shift while adjusting my position before a jump. Since you move at normal walking speed, I almost always miss the jump.
To reproduce
According to this comment by @unknown, pressing the W key three times consecutively can cause this issue.
Linked issues
is duplicated by 25
Comments 63
Duplicate of MC-1021.
Yes, I know it is a duplicate. I was giving a better description than "the FOV went boingoboingoboingo", and ways to replicate it.
I do use mods, but I tried replicating it using a plain vanilla client. I tried in various ways, it only seems to occur on internet servers. I could not replicate it at all in single player, or on a locally based server. I connected to a random vanilla server, and as soon as I found land, I was able to easily replicate it. You have to start sprinting within a split second of letting go of shift.
I recorded a video, to show the bug. It took a couple tries at the start, since my FPS was decreased a bit from recording. In the second half of the video, I was showing that you do move at normal walking speed by making a jump that is only possible while sprinting without the bug.
I have never heard of this before, nor can I reproduce it. Do you have any mods installed? Does this still occur if you update your java (your version is very outdated)?