ON a Mvvs 58cc gas engine i have, my problem is, when i put engine at full throttle and leave it there for 45 to 60 seconds or so, all of a sudden the rpms drop by 400 to 500 rpms.
So I slow down the rpms to about idle and then throttle up to full, back and forth ,till I eventually get all rpms back at full throttle.
Normal rpms at full throttle are 6900rpms using a 22 x 10 wood Menz Prop.
Can you help me???

How old is your engine?
What make of ignition is on your engine?
Do you use proper fuel? (With 1:40 oil mix) and what oil quality?
Muffler system used?
Is it in flight or on a test stand?
How are your needles tuned?
Is the front seal lubricated?
Is the vacuum pulse connection to the carb still OK?
Cooling issues?
Is the battery voltage that reaches the ignition stable?
Plane failsafe function?

the engine is about 5 yrs old, but piston and ring and don't know what else was replaced because of a defect. this was done in chezh.
ignition is a vlach electronic ignition EZM 1 TI
Using 93 octane gasoline with Klotz 2 stroke oil at 1:50 ratio. synthetic with castor
Muffler is a Bisson
engine is on the plane, and has the problem both in flight and on the ground.
needles are set at 2 1/4 turns on the high end needle and 1 3/4 turns on the low end needle
i do not know if front seal is lubricated, how can i tell?
vacuum pulse connection to the carb is ok.
seems like when i turn engine off it takes a good while before you can keep your hand on the engine crank case.
battery voltage stable
plane failsafe is set off.

A few things come to mind, maybe they are combined.
1) The engine should be lubricated 1: 40, because of seal lubrication. A leaking seal can influence the running of the engine, especially when hot. To keep the front seal happy, a drop of oil applied externally now and then works miracles. Klotz is not one of the preferred oils (R50 is, but not at 1:50). If the seal has too much resistance (sometimes it even groans), it is starving for oil. Once you see black oil traces in/on the front housing it is too late.
Check out http://mvvs.nl/ left column "petrol engines" , scroll all the way down to see the oils listed in bold.
2) The engine crankcase should not get too hot. This also is detrimal to the front seal life expectancy.
3) When the case gets too hot, the small line from crankcase to carb that takes care of carb pumping action becomes softer and may start to leak, preventing engine full power. Check the many forums on engine cooling and baffling. Gas engines need air baffles to stay healthy.
4) The ignition, If mounted in the cowl, may get too hot.
5) The ignition may start acting up and needs to be replaced (check all leads for damage)
6) The carb may have some roaming debris inside.
7) Fuel lines from the tank may develop cracks.

I would suggest, you give your plane and engine a thorough checkup.