Rear derailleur slipping under pressure

Just a little help needed before I tinker with my derailleur

So I replaced my rear derailleur cable on Monday, no problems, shifts up and down perfectly looks all in line etc.
degreased my chain gave it a good old scrub. left it over night then before I went out for a ride yesterday I used some pure wet oil on the chain, ran the chain up and down the cassette to get in all the teeth then went out for a ride (I forgot to wipe off the excess oil from the chain before leaving the house) about ten miles into my ride my chain started slipping when in the highest sprocket when I used any force uphill. headed home cause it really annoyed me and took a screwdriver to my stop screws, also messed with the cable tension by turning the hose spring at the back of the arm. tried it again and it's still the same. degreased the chain-set this morning before I went to work and I'm going to sort it this evening when I get home. anyone got any ideas what else it could be?
It's a golden arrow rear derailleur.
the chain is brand new
the cassette teeth are not worn.
it only started slipping after I greased the chain
too much oil?
needs altering (stop screw adjustment)
cable tension adjustment?

Mmmm, dunno. My one has the same symptoms but not all the time. How tight are the shifters? I take it they are friction? If there isn't enough friction and the cable slips slightly then that, when combined with a little frame flex as you power up the hills could be enough.

Mine has been a lot better since I tightened the shifters.

I thought about that, yeah they are friction and after replacing the cables I did notice they were a bit loose.

When you say the highest sprockets, do you mean the largest or the smallest?

Hi,

How was the new chain connected - did you break the chain to put it on, possible stiff link - my 2p.

Richard

new chain has been on there a month, no problems, only since I replaced the rear cables, and I'm meaning the largest sprocket, so when I shift down to easiest gear for climbing, it jumps straight out of place if I push hard. really annoying

And you checked your drop/hanger for alignment? it´s the most important thing to do, whenever you mess with your gear system. I work as a Team mechanic, and build a lot of bikes from bare frames, no matter how expensive a frame is it always needs to be aligned, I have never found one that was straight.

Sounds like an adjustment issue. As you know if the chain has to "stretch" to get into that big cog then there is always a chance it will slip because it is not fully engaged. Re-check the limit screw for the big cog?

Steven

All sorted, was a combination of a slightly loose shifter and slightly out of line arm. Thanks for the help. As always