Be aware that if you are using an instruction to do a read/modify/write
operation on a port then you need to have at least 1 instruction between this
instruction and the previous instruction that wrote to the same port. The reason
is that the write to the port is done at the tail end of the operation, and if
you then do a bit operation (this is a read/modify/write type instruction) then
the read of the port status is done very early in the operation. It may be that
you have problems because the level on the pin has not had sufficient rise time
to get to the state you expect on this next instruction.

Also be aware that when an I/O port is read, the actual state of the pin is
read, not the state of the output flip flop. The result of this is if you do a
read/modify/write operation on the port, if you have the base of a transistor
directly attached to the port, then the pin may not get to a high enough voltage
to register as a "1" state.