I think this one needs a slight change in wording, because right now it seems to go off in a different direction halfway through. I’ll highlight the differences in red so you can see what I mean.
They had shifted so many times in the last few weeks that now they were getting weak.
They had shifted too many times in the last few weeks, and now they were getting weak.
Notice that the second version has a comma after weeks, but the first doesn’t.
If you’re phobic of the word that (because you listened to someone who parrots “rules” without understanding them), you can leave it out but add a comma, and the meaning of the sentence will remain more or less the same, just rather informal.
(WordPress’ spell check tells me red is misspelled in this post. *shakes head* This, kids, is why you can’t trust a computer program to do a human-ish person’s job when it comes to editing/proofreading.)