Problem reading flowmeter pulse in PCS7

I have a little problem when reading the flowmeter pulse for counting total quantity through the flowmeter. I use PCS7 v7.1 SP1 and flowmeter of E&H manufactuerer.

I've set the pulse width of Flowmeter is 100ms, in my program, I use a CH_DI block as input and an own block for counting quantity of water through the flowmeter after recieve the rise pulse of Input (0.1hl/pulse). My problem is the counting value sometime right, sometime wrong, sometime it count to a number and stop count,even thought I put those blocks in OB100 or OB1, OB36, OB37, OB38... the result is not improved. The customer complain me a lot, but I can't find a better way to fix this error. After I change the OB of blocks, it work fine in some days and then returns to old case.

Do you have any ideas to help me solve this problem? because if the count value not correct,I cannot change to next step correctly.

Thanks you for all your ideas.

But also the idea of moving to a PIP and assigning the PIP to the OB35(100ms) is a good attempt (also cheaper)

I discovered the problem and solved it. The reason is the noise on signal cable from the sersor to PLC. I change another cable and made it more saperate with another cable, and the signal is correct.

Thank you all for your support.