HP15c runs slow

[Adalin]

Hello

I have two hp15c calculators, one made in Brazil 1988 and one made in USA 1985

But I have a small problem with my Brazil one, it runs much slower than my USA made one and I cant figure out why.

Sometimes when I try to start my Brazil one the ON button does not work, it does not react at all.
I have to press ON and ÷ to start the keyboard test, then I can press Enter to abort it with error 9 and the calculator works again, and sometimes when I press f and clear ∑ the screen goes blank for a long time before I can use it again.

The same sometimes when I press a number and SIN COS TAN the screen goes blank for a long time before presenting the results.

I know that HP15C is a slow calculator in general but I dont have these problems with my USA calculator at all, and its irritating when I use it and things take longer than expected.

but its not always like this, many times it works as normal.

It performs the ON X selftest with no errors and I have tried to replace the batteries.

what could cause the calculator to run slower than normal?