Quatrain 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Sometime all full with feasting on your sight,

Are you starting to notice a pattern here? Back in Line 5, the speaker began by describing the miser as "Now proud as an enjoyer." A little later, in Line 7, the speaker described himself as "Now counting best to be with you alone."
In each of these two cases, the initial good situation turned into something else, something bad. At the end of Line 5 and in Line 6, the miser's confidence turned into fear that someone else would steal what he had. Then, in Line 8, the speaker's own confident enjoyment of his relationship turned into the anxious desire for everyone else to be able to see it.
In Line 9, the same pattern seems to be at work. The word "Sometime" at the beginning of the line means just what it looks like: "At one time." But what happens "at one time"? The speaker is "full with feasting on your sight."
So, is this a continuation of the imagery that has already come before, or is it new imagery?
We'd say that depends on your perspective. This is new imagery in one sense. The speaker has turned from the "miser" imagery from Lines 4-6 (which is arguably foreshadowed in Line 3 and arguably continues into Lines 7-8) and replaced it with imagery derived from "eating."
But is this "eating" imagery really new? Didn't the speaker tell us way back in the first line of the poem that "you" are to his thoughts what "food" is to "life"?
Why would he repeat himself? Could he be using this eating imagery in a new way at this point in the poem? Let's keep reading to find out.

And by and by clean starvèd for a look;

Now we're starting to see what Shakespeare is up to. First of all, he is following the same pattern here that he followed in Lines 5-6 and 7-8. He starts off by describing a good experience and then shows how that experience turns into something bad.
Here, the good feeling of being "full with feasting" from Line 9 turns into the bad feeling of being "clean starvèd" (i.e., completely starving) in Line 10. (Note: that accent on the last syllable of "starvèd" is just supposed to make you pronounce it as two syllables: STAR-ved. For information on why Shakespeare's meter makes this necessary, check out the "Form and Meter" section of this module.)
At the same time, we see how Shakespeare has taken the positive "food" imagery from Line 1 and shown its downside. Sure, food is great when you have it, but when you don't have it, it's bad. It's starting to look like the speaker's relationship brings him sadness as well as happiness.
Once again, it looks like these lines are referring to the experience of the speaker, continuing the pattern begun in Line 7.

Possessing or pursuing no delight

Line 11 continues to refer to the speaker. Here he describes himself as not having any "delight" and not "pursuing" any. Based on what has come before, this sounds pretty weird. We thought this guy was describing himself as filled with "strife," as sometimes "full with feasting" and sometimes "clean starvèd."
Has the speaker suddenly turned all puritanical? Let's keep reading to find out.

Save what is had or must from you be took.

Ah, now things are clearer. It isn't that the speaker isn't "possessing or pursuing" any "delight" (Line 11). He's just only interested in what he can get from "you."
Just so we're on the same page: the word "Save" at the beginning of this line is being used in an old-fashioned way where it means "except." Also, you're supposed to understand the rest of the line as meaning that the speaker is interested in (A) "what is had from you" and (B) "what must be took from you."
But wait: when the speaker says that he is interested in what "must from you be took," what does that say about "your" feelings about this relationship? Do "you" not consent to the speaker's relationship with "you," at least some of the time? That might explain the highs and lows in the speaker's attitude—though modern readers are likely to be uncomfortable with the speaker's hint that he sometimes "takes" things from "you" that "you" don't want to give.