Screen corruption & possible crash (*solved*)

[LCD]

I wrote a function to transform Speccy frames to a timer:

Code:

function Clock(tim as uinteger) as String
    dim secs as Uinteger
    dim mins,sec as ubyte
    dim s$ as String
    s$=""
    secs=int(tim/50)
    mins=int(secs/60)
    sec=secs mod 60
    s$=str(sec)
    if len(s$)=1 then
        s$="0"+s$
    end if
    return str(mins)+":"+s$
end function

dim a as uinteger
for a=0 to 1000
    print at 0,0;Clock(a)
next a

After calling it often (it shows effect around timecode 0:08), there was a screen memory corruption. In my full program the timer was still working, but the keys did not work anymore, and after that, even the minute display was corrupted. Maybe a overflow?
Reducing the Heap size from default to 100 caused no corrupted screen memory, but a complete crash.
Oh, and I'm using the latest devel version.

Edit:
It looks like there is a leak in the STR function, as without STR my program does not crash after a while.