Couldd be your cat mate sounds like it could be loose and when you go up hill its moving and restricting the exhaust gas flow I had a similar problem with my escort

Found the problem (I think!). I have been wanting to change the spark plugs myself as they are due to be done, but have put it off due to horror stories of cracked heads from rusted and seized in plugs. This is caused by leaking washer jets flooding the plug recesses (common problem on this model). Mine had this problem as the previous owner told me. My local garage said they would change the plugs, but were not happy and said it was at my own risk. They showed me a cracked head from one they tried before! Anyway, I thought I would have a go and when I pulled the ignition leads from the plugs, I found that they were all swimming in oil! This scared me, but I decided to try and some how get the oil out. I used a combination of kitchen turkey basting syringe (best thing ever for this) and rags pushed into the holes to soak. Anyway after a good half hour or more of cleaning, I felt happy enough to have a go at removing the plugs. They all came out nicely, but there was rust residue on all of the threads. The tips were all nice though and nothing had leaked past the threads. I cleaned the threads up and cleaned the ignition lead ends as best I could as these were wet with oil. God knows how the car even started like this! Anyway, all back together and the car runs so much better. The flat spots seem to have gone and it starts every time without issue. The oil does not seem to have come back (so far), so I don't think it's the rocker cover gasket. Maybe the previous owner/garage chucked some oil down the plug holes (too much) to make removing the plugs easier? Strange. Should I bother putting new ignition leads on?