I have virtually the same model, an TS1610F, with the same Isuzu motor. And I had the exact same problem as yours, except for the hard starting part. The problem is most likely worn rockers (4-ea.) and rocker shaft. After replacing a lot of top end parts on mine, including rings, rod bearings, valve seals, and head gasket, came to find out it was the rockers passing too much oil to the top of the cylinder head, under the valve cover. It literally filled up with oil because it couldn't drain back down the pushrod tunnels to the oil sump fast enough. Confirm this is the problem by taking the valve cover off and start the motor, then look for excess oil flow from a couple rockers. This problem has been reported by a few others so is somewhat common for some reason. You're looking at probably $300.00 in parts (4 rockers, one rocker shaft, and a head gasket). Contact DESSCO.com and talk to Jim Hiner in parts. He'll need the engine model No. 2AA1 and a spec number, he should be able to give you that. Good luck,
Bill