Holden Combo loses power

The 1996 Holden combo I picked up cheap with 190,000 km on the clock seems to have power issues. When I accelerate I either lose power or it feels like I'm towing a 100-tonne excavator. I've replaced the exhaust, the plugs and leads, the throttle body, map sensor, fuel filter, oil filter and everything I can think of. I've also checked the fuel pump and it's working fine. I'm about at a dead end.