When you do the seals in the oil pump make sure you replace the shaft seal. The cam wheel can be hard to remove to get to the seal but it's part of the system and is known to suck air. If you only replace the "mickey mouse" seal you're only doing half the job. Also check the shaft for wear, if there's a groove in the shaft replace it.
Another little trick I've used with relatively good results is adding a 2-3mm spacer behind the pressure relief valve (ball and spring arrangement). Further to this, if you can be bothered since the engine is still in the vehicle, you can clean out the oil passages in the head and have a look at the area in there - I'm 99% sure there's another pressure relief setup on each head, if the spring is jammed it's not going to allow oil to pressurise in that area which could lead to dodgy lifters.
If your lifters have been doing this for a long time they could be worn to the point that they won't oil up and hold the pressure due to tolerances becoming too large with the constant movement. In this case it is best to buy new lifters or "look into an EJ conversion"
I had to slip that last bit in there somewhere
Cheers
Bennie