If you have established on stands that the transmission and all wheels are moving freely, this suggests that the engine is not capable of producing a reasonable amount of power when it is required. (possible the handbrake is not always releasing fully)
Assuming it is a four cylinder engine, likely problems are -
Throttle is not opening fully
Incorrect timing, ignition more likely, but valve timing possible. Distributor automatic advance perhaps seized.
Poor fuel delivery - air leak on suction side of pump, faulty pump, restriction between pump and carburettor, blocked jet etc
Intake manifold air leak
Ignition issues - points gap too small, faulty capacitor, faulty coil, broken earth wire inside the distributor that breaks the connection when the automatic advance operates (this more or less hidden)
That is about what I can think of at the moment.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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