Gearbox flaring

Hi guys, ok been home for a few days and after the last oil change in the box with lucas added my box now only flares when really cold or when using kickdown from 4-3 and then flares for about a second changing back up. If I drive with 3 selected I can gun it from a standing start and get crisp gearchanges then slip it into D and its fine, when overtaking if I manually select 3 and floor it there is no flaring and then goes into top with no drama when slipped back into D. I did not get the time to drop the valve block last time but hope to this time, I have googled to reserch the box but can,t find if there is a gaskit needed between the valve block and the box and do I really need to use the setting tool. Trying to find a 200 psi gauge at the moment, havn't looked but take it the thread on the orifice is npt. Driving normally I get no flaring in D, only when giving it some welly. I have seen a master rebuild kit for £200 but don't really want to tear the whole box down. Thanks in advance for any info.
Cheers
Davie

Took the bull by the horns and dropped the valve body's today, did find one kind of sticky, cleaned them up, re-fitted and filled with dextron 11, as its the 3 oil change in as many months i thought i would fill it with the cheaper stuff first and if all good change it later. Car seems a lot better, have a little surge for about a second rather than flaring on kickdown but its only the first day so we will see what happens, funny though car seems smoother with dextron 11, might be my imagination. Oil that came out seemed very thick, putting that down to the bottle of Lucas additive, it was also very dirty again which is a major worry.
Davie