Sounds like the starter motor isn't receiving enough current. See if you can run a wire directly from the relay on the ignition side to the solenoid, under the bonnet.

As wires get hotter, their resistance increases. I had this problem on my last car with a Rover V8 engine. Solved it by taking the original solenoid wire to a relay, and then putting a new lead onto the solenoid - from that same new relay. Relay was then connected directly to the battery. So basically using the original wire in the old loom to switch the new relay, which then provided a nice fat full 12V and plenty of current to the starter. Worked a treat.

cool differences to American English
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!