Forgot to update, it ended up being my distributor. The centre pin was corroded and not making a good connection.
Since then I've replaced the timing belt, water pump, belts, hoses in trying to get it up to snuff to pull our "new" tent trailer. Just did the tensioner rod (runs from lower a-arm to frame), it had worn an egg-shaped hole in the frame and was making lots of noise and hopefully it'll be good for a while!
320k and still going sort of strong.