What is the deal with my car?!! (1985 300d)

[soulbean]

OK. I have adjusted the valves, changed the filters, put biocide in the tank, added a HHO maker, and the car has the most power it's had since I bought it this past summer for $640. It halls. Big time. Consistantly. Except for the following scenario:

The problem I have is the following: EVERY time I drive the thing for an extended period of time, (with excellent power experienced, I might add), and I stop, turn the car off, and run an errand, EVERY FRICKEN TIME, the car struggles to start. When it does, it has NO power. I usually can limp home... The next day, if I have to go from point A to point B, I will have excellent power. If I have to stop and turn the thing off, it's slow mobile-ville again.

PLEASE HELP!

(BTW, this SAME PROBLEM existed BEFORE the valve adjustment and BEFORE the HHO maker and BEFORE the filters where changed. I just have more power now, but the same initial problem persists...)