I think, you did not understand what I meant. I wasn't referring to the overall hashrate of the whole clevermining-pool including all users, but the single hashrate of one user.
Why is it dropping, if you count each and every submitted share even of the previous mined coins, after the pool switched? If it was like this, the hashrate could not drop from let's say 80 MH/sec to 60 MH/sec. for some time (sometimes 5 minutes or more), right?
Then I think you don't understand what is hashrate reported by the pool. Let me explain.
The pool doesn't know your hashrate. It can only estimate it based on number of shares which your miner submits, so what the pool shows is only an estimation. You miner knows your exact hashrate because it knows how many hashes it calculated. But it only sends to the pool hashes that meet required difficulty (they're called shares) and the pool estimates your hashrate based on these submitted shares (based on knowledge than it takes X MH/s to find one Y-difficulty share per second). But number of shares which your miner finds varies wildly as mining is a kinda-random game. Sometimes you find 10 shares in a minute, sometimes you won't find even a single share in a minute. If you look at 5-minute values then your hashrate estimated based on shares submitted can jump between 60 and 80 MH/s and even more. The shorter the timespan the bigger the variance. Just like in throwing a dice. If you throw a dice three times and look at each result individually then you can get 1, 3 and 6. If you throw 30 times and take 10-throw averages then you can get 3, 3.6, 4.1. If you throw it 300 times and take 100-throw averages then you can get 3.4, 3.5, 3.6. It's the same with checking 5-minute, 1-hour and 24-hour mining averages. What you want to compare is 24-hour average, anything less is just too vulnerable to variance.