I remember deciding a long time ago to instantly dismiss people who use the word “uhhm” while gathering their thoughts. I’m not saying I’ll refuse to deal with them — there are tens of thousands of “uhhm”-ers out there and you can’t live on an island. But the instant I hear that word I tend to pull back and regard the speaker askance. You’ll notice that for the most part “uhhm” is used by younger people and under-educated girly-girls.
I can roll, on the other hand, with people who say “uhhh” or “ahhh.” (I’ll occasionally resort to these.) It’s really the use of the letter “m” that tears it. When you’re pausing between phrases, just don’t say anything that rhymes with the word “bum” — simple.