Drop your SC throttle down to 50%. Then once your speed stabilizes, hit the Interdict button. This will give the best possible maneuvering characteristics for your ship.
Once a tether is established, vary your throttle. 50% if you out of alignment and need to turn quicker, then bump it up to 100% if you are aligned with your target.
It takes a little practice, but once you master it, no NPC will escape you. Unless they're Elite, and in an agile ship, while you're in a oversized combat tug.
This in a nutshell is how I Interdict someone. The only thing I would add is to constantly roll your ship while interdicting. This seems to be effective as well.
Evading interdiction on the other hand............
90% of the time you are in SC you're probably at 100% throttle. That is when people usually get interdicted. Of course it will seem impossible to escape. So really you have 5 choices while in SC:
1) drop to 50% throttle as quickly as possible and try to fight it
2) try to outrun them... which only works if you already have the speed advantage and you're not trying to dock anywhere.
3) submit and fight/evade/stealth/flee/etc
4) do an emergency drop to SC before they can tether you
5) If you've got a good lead, use a gravitational body to slow them down if they try to directly pursue you. Asteroid Clusters work great since even Players won't see them.
That's not really the problem. Last week, when I came up from behind interdict them, I could get connected to them and I could pull them out of warp to make the kill, and I was killing eight out of ten of them. Now, I'm averaging one out of ten. Something has changed. What I've been doing lately is flying by planetary systems to let them pull me out of warp to make the kill. Which takes much longer to do. Have them changed the program, or does anyone know?
Pre 1.4 interdiction killed my fun, now I'm having fun, since I actually have to pay attention.
They rebalanced interdictions, on what factors, nobody knows except Frontier. That has killed my PowerPlay ranking as well, I was barely able to reach the 5K merits each week to maintain my rank 5. Right now, I have about 2K and still 3K to go in the next 3 days. I'm not going to make it, not by a long shot. I make about 200 merits each day by delivering fortification documents. The rest has to come from undermining but, like you, with 1 successful interdiction in 10, I don't have the time to get all the merits I need, simple as that.
Sometimes it feels as if they take some decisions based on the assumption that a player has nothing else to do than play E: D all day. Anyway, i guess it's difficult to keep balance between players playing 1 hour / day and players playing 6 hours. That's one reason i'd prefer to have a truly offline version. You are the only one that the game has to have "in mind". It could either have an adaptive automatic system to adjust its aggressiveness according to your skills or a manual "difficulty" selector.
Yesterday evening, for the first time, I had a 50% success in interdictions. What I did was a bit more drastic than what you are suggesting. In my Python, I would immediately cut the throttle when the interdiction would start. Might be a fluke, maybe my network connection was better but I'm definitely going to try it again tonight. Might worth a try, at least in a Python. I have a 3A FSDI, for those interested.