Just thowing this thought out there because you stated you did the seals already..If you valve guides have excessive clearance, I believe that will lead to premature Valve seal wear, as the valve will move around more side to side in the guide, stretching out the seals and their little spring in the process. Just a thought..Like eddy said it could be a number of things.

99 Gen 2.5, fixing blown head gasket
89 SWB- 33's, ARB Front locker, SR rear locker/axle, SR F brakes, winch, WST Offroad Armor all Around, 2.85 Aussie T-case Gears (SOLD)
Sold: (2) 95 SR's, 86 SWB, 90LWB, 91 LWB
-Can Change a timing belt in my sleep..