Ok, I have to admit, if I'd paid more attention to Electronics, and less
attention to Programming, I probably wouldn't be asking these questions...
but...

Exactly what are the rules for combining 74LS, 74HCT, 74F etc... in the same
circuit?
In the past, I've only built small circuits, and it hasn't seemed to matter too
much.
However, I'm getting some *very* strange behaviour from a PIC16C64 project,
and I'm beginning to suspect it's something to do with current, or perhaps
capacitance, or one of those quantities I know nothing about (spot the
programmer!).

The circuit is a PIC16C64 @ 20MHz.
Pins A0-A2 to go S0-S2 on a 74LS138N inverting demultiplexer. Pin A3 is the
ENable on the demux. The 8 outputs from the demux go to a bunch of different
peripherals (LCD, SRAM, etc... to control reading/writing etc...).
One of the demux outputs (03) goes to LE on a 74LS574N octal D-flip flop, as
well as going through a 74F04 hex inverter, and then on to /EN on a 74HCT245N
bus transceiver.
Pins B0-B6, C0-C7, and D0-D7 go to all the peripherals. C0-C7 go to the
flip-flop, and D0-D7 go to the bus transceiver.
Basically, I use A0-A2 as a device select, and A3 as an "enable", for
reading/writing to SRAM, IDE interface, and the flip-flop/transceiver which are
hooked up to a PC parallel port.

Most of the time it works just fine. However, when communicating with the PC,
if I hold a logic probe on the demux O3 output, I start getting comms errors.
I've checked all the connections, and they look good...

I'm tearing my hair out on this one!!! Any help would be GRATEFULLY received.

Cheers,
Ben