I had this problem on a 97' K1500 w/ 115,000 milies on it. I think you are
describing what is known as a "Bang 1-2" problem. I put off fixing it because
the problem would come and go - similar to your discription. However, now I
wish I had not put it off for so long. I ended up toasting the clutch. I (my
mechanic from a referral in this group) opened the valve body and it did not
help. We ending up rebuilding the entire trans.
One word of advice, get it fixed quick.