ORIGINAL: Rodney
You might experiment a bit with changing the thrust line, increase the downthrust a bit and see what happens. It may be that you could also move the CG a bit by moving the battery forward or aft and see if there is a change.
You might try Rodney's suggestion by placing washers between the firewall and the top of the engine mount. This will reduce the up thrust (increase down thrust).
DaveB