Slip,

> After a few seconds of use, at random, the LCD will start misbehaving - 
> somtimes the screen blanks, often it displays random characters

Does it do this by itself or is it corrupting data you send ?

> To get it working again, I have to reset my circuit

Are you resetting the PIC or the power ?

> I'm only a hobbyist and i'm struggling to work out what the cause is

Do you have any access to an oscilloscope ? Can you put LEDs on the
LCD lines to detect any unwanted activity, particularly Enable, which is
the only line which signals the LCD to process data

> I've tried adding a decoupling cap at the power pins of the LCD module

I've noticed that some LCDs won't initialise properly without a decoupling
cap but run OK otherwise. It doesn't sound like an initialisation problem

> The circuit (pic and lcd) are powered from a 7805 1 amp

Is the regulator's input and output decoupled ? It might be oscillating at
some point

> I've tried massively increasing the time delays

If it really is 44780-compatible then commands should take 40us for most.
Set-up times as short as 1us should work

> I'm not sure whether its a software or electrical problem

Is there anything else in the circuit like a relay or solenoid or something
that might cause power spikes ? That can make an LCD go crazy
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