That can be due to other things. When I had the 32in1 OS installed I'd get random glitches in characters. Supposedly the problem can be due to Phi2 timing issues and also due to certain sequences of Ram/Rom accesses where one or the other puts residual data on the bus for too long.
I always change out U18 74LS08 for 74HCT08 every
chance I get and my babies seem to love me for it.
Randomness in screen characters goes away, I like
to think it's because of the more square Phi2 clock
but I don't have a means to look at it yet. I can
swap out a chip though. It's not a common problem
of mine, but I have seen it several times. MMU
might be involved too, this sort of smells like
the CPU at the same time especially the warm up
time part. At least that's my take on it.